Halacha

הלכה א
הַשׁוֹחֵט חַיָּב. וְלֹא שׁוֹחִט בִּלְבַד אֶלָּא כָּל הַנּוֹטֵל נְשָׁמָה לְאֶחָד מִכָּל מִינֵי חַיָּה וּבְהֵמָה וְעוֹף וְדָג וְשֶׁרֶץ בֵּין בִּשְׁחִיטָה אוֹ בִּנְחִירָה אוֹ בְּהַכָּאָה חַיָּב. הַחוֹנֵק אֶת הַחַי עַד שֶׁיָּמוּת הֲרֵי זֶה תּוֹלֶדֶת שׁוֹחֵט. לְפִיכָךְ אִם הֶעֱלָה דָּג מִסֵּפֶל שֶׁל מַיִם וְהִנִּיחוֹ עַד שֶׁמֵּת חַיָּב מִשּׁוּם חוֹנֵק. וְלֹא עַד שֶׁיָּמוּת אֶלָּא כֵּיוָן שֶׁיָּבֵשׁ בּוֹ כְּסֶלַע בֵּין סְנַפִּירָיו חַיָּב שֶׁעוֹד אֵינוֹ יָכוֹל לִחְיוֹת. הוֹשִׁיט יָדוֹ לִמְעֵי הַבְּהֵמָה וְדִלְדֵּל עֵבָּר שֶׁבְּמֵעֶיהָ חַיָּב:
כסף משנה
1.
A person who slaughters is liable. This does not apply only to [ritual] slaughter. Anyone who takes the life of a living beast, an animal, fowl, fish, or crawling animal - whether by slaughtering, stabbing, or beating1In all three of these activities, the animal dies because of bleeding. For this reason, although beating may cause the animal to die because of internal bleeding, it is included in the same category. - is liable.
A person who strangles a living creature performs a derivative of slaughtering.2Since no blood is shed, this is considered a derivative and not a מלאכה מעין. Therefore, if one removed a fish from the glass of water [in which it was being kept] until it died, one is liable for strangling it. [Indeed, one is liable even if one returns it to the water before] the fish actually dies. As long as [a portion of its body as wide as] a sela3A coin of the Talmudic period. Based on Yoreh De'ah 30, it appears that a sela is approximately 2.6 or 3.2 cm in diameter according to the different opinions. between its fins becomes dry, one is liable, for it will not be able to live afterwards.
A person who inserts his hand into an animal's womb and removes a fetus [from] the womb is liable.4Based on Avodah Zarah 26a, the Eglei Tal quotes opinions which state that if a person performs such an act, he is liable for reaping - i.e., removing an entity from its source of nurture. The difference between these two rationales is that when an animal is prepared to give birth, the concept of reaping no longer applies, for the fetus no longer needs its mother's nurture.

הלכה ב
רְמָשִׂים שֶׁהֵן פָּרִין וְרָבִין מִזָּכָר וּנְקֵבָה אוֹ נֶהֱוִין מִן הֶעָפָר כְּמוֹ הַפַּרְעוֹשִׁין הַהוֹרֵג אוֹתָן חַיָּב כְּהוֹרֵג בְּהֵמָה וְחַיָּה. אֲבָל רְמָשִׂים שֶׁהֲוָיָתָן מִן הַגְּלָלִים וּמִן הַפֵּרוֹת שֶׁהִבְאִישׁוּ וְכַיּוֹצֵא בָּהֶן כְּגוֹן תּוֹלָעִים שֶׁל בָּשָׂר וְתוֹלָעִים שֶׁבְּתוֹךְ הַקִּטְנִיּוֹת הַהוֹרְגָן פָּטוּר:
כסף משנה
2.
A person who kills insects and worms that are conceived through male-female relations or fleas that come into being from the dust is liable as if he killed an animal or a beast.5This follows the Rambam's ruling that one is liable for performing a מלאכה שאינה צריכה לגופה. The authorities who differ on this principle will hold one liable only if one kills these creatures for the sake of something that one needs from their bodies. In contrast, a person is not liable for killing insects and worms that come into being from dung, rotten fruit, or the like - e.g., the worms found in meat or those found in legumes.6In view of the experiments of Pasteur, there are many Rabbis who maintain that this ruling should be disregarded and we should refrain from killing any creatures on the Sabbath. Others, even contemporary authorities with scientific backgrounds, such as the Lubavitcher Rebbe Shelita (see Emunah UMada, p. 130 ff), maintain that our inability to observe spontaneous generation is not absolute proof that such a phenomenon does not exist. Consequently, it is inappropriate for us to think of altering the halachah.
In this context, it is significant to quote the Rambam's statements, Hilchot Shechitah 10:13:

Similarly, with regard to the conditions that we have enumerated as causing an animal to be trefah (unable to live for an extended period): Even though it appears from the medical knowledge available to us at present, that some of these conditions are not fatal... all that is significant to us is what our Sages said, as [implied by Deuteronomy 17:11]: "[You shall act] according to the instructions that they will give you."

See also the introductions to Chapter 3 of Hilchot Yesodei HaTorah and to Chapter 4 of Hilchot De'ot in the Moznaim Mishneh Torah series.

הלכה ג
הַמְפַלֶּה כֵּלָיו בְּשַׁבָּת מוֹלֵל אֶת הַכִּנִּים וְזוֹרְקָן. וּמֻתָּר לַהֲרֹג אֶת הַכִּנִּים בְּשַׁבָּת מִפְּנֵי שֶׁהֵן מִן הַזֵּעָה:
כסף משנה
3.
A person who checks his clothes for lice on the Sabbath may rub off the lice and discard them.7See Shulchan Aruch, Orach Chayim 316:9. It is is permitted to kill lice on the Sabbath, for they come into being from sweat.8The difference of Rabbinic opinion mentioned with regard to the previous halachah applies in this instance as well.

הלכה ד
חַיָּה וְרֶמֶשׂ שֶׁהֵן נוֹשְׁכִין וּמְמִיתִין וַדַּאי כְּגוֹן זְבוּב שֶׁבְּמִצְרַיִם וְצִרְעָה שֶׁבְּנִינְוֵה וְעַקְרָב שֶׁבַּחֲדַיֵיב וְנָחָשׁ שֶׁבְּאֶרֶץ יִשְׂרָאֵל וְכֶלֶב שׁוֹטֶה בְּכָל מָקוֹם מֻתָּר לְהָרְגָן בְּשַׁבָּת כְּשֶׁיֵּרָאוּ. וּשְׁאָר כָּל הַמַּזִּיקִין אִם הָיוּ רָצִין אַחֲרָיו מֻתָּר לְהָרְגָן וְאִם הָיוּ יוֹשְׁבִין בִּמְקוֹמָן אוֹ בּוֹרְחִין מִלְּפָנָיו אָסוּר לְהָרְגָן. וְאִם דּוֹרְסָן לְפִי תֻּמּוֹ בִּשְׁעַת הִלּוּכוֹ וְהוֹרְגָן מֻתָּר:
כסף משנה
4.
It is permitted to kill beasts or insects whose bites are surely deadly, as soon as one sees them9Shabbat 121b explains that this leniency applies even according to the opinion of Rabbi Yehudah, who maintains that one is liable for performing a מלאכה שאינה צריכה לגופה. Because of the danger these species pose, they are allowed to be killed even when they are not pursuing a person. For it can be assumed that as soon as they become aware of a person, they will pursue him. - for example,10The Rambam cites these examples to illustrate the principle he states, not to be restrictive. If other species pose mortal danger in a manner similar to the five mentioned here, the same laws apply (Kessef Mishneh). flies in Egypt, hornets in Nineveh, scorpions in Adiabena, snakes in Eretz Yisrael, and rabid dogs in all places.
[The following rules apply with regard to] other dangerous animals:11I.e., creatures that can inflict mortal wounds, but will not necessarily do so. If they are chasing a person, one may kill them.12In this instance as well, Rabbi Yehudah would accept this leniency, for the person is concerned only with saving his life and not with the performance of the forbidden activity. (See the interpretation of Sefer HaBatim mentioned in the notes on Chapter 10, Halachah 17.)If they are staying in their place or fleeing from the person, it is forbidden to kill them.13For there is no immediate danger involved. If one steps on them accidentally as one is walking and kills them, this is permitted.14The wording used by the Rambam (based on Shabbat 121b) is somewhat problematic. The expression "if one steps on them accidentally," implies that we are speaking after the fact, after one has already crushed them, while the expression "it is permitted" appears to indicate that this license is granted at the outset.
The Rashba explains that, indeed, license to kill these creatures is granted outright. One should merely make it appear that one is stepping on them accidentally. This ruling applies, however, only according to Rabbi Shimon, who does not hold one liable for performing a מלאכה שאינה צריכה לגופה. According to Rabbi Yehudah, the intent is that one may proceed without taking care to check whether one kills them or not.

הלכה ה
הַמַּפְשִׁיט מִן הָעוֹר כְּדֵי לַעֲשׂוֹת קָמֵעַ חַיָּב. וְכֵן הַמְעַבֵּד מִן הָעוֹר כְּדֵי לַעֲשׂוֹת קָמֵעַ חַיָּב. וְאֶחָד הַמּוֹלֵחַ וְאֶחָד הַמְעַבֵּד שֶׁהַמְּלִיחָה מִין עִבּוּד הוּא וְאֵין עִבּוּד בָּאֳכָלִין. וְכֵן הַמּוֹחֵק מִן הָעוֹר כְּדֵי לַעֲשׂוֹת קָמֵעַ חַיָּב. וְאֵי זֶהוּ מוֹחֵק זֶה הַמַּעֲבִיר שֵׂעָר אוֹ הַצֶּמֶר מֵעַל הָעוֹר אַחַר מִיתָה עַד שֶׁיַּחֲלִיק פְּנֵי הָעוֹר:
כסף משנה
5.
A person who skins [a portion of an animal's] hide large enough to make an amulet is liable.15This is one of the 39 categories of forbidden labor. Similarly, one who processes [a portion of an animal's] hide large enough to make an amulet16The Maggid Mishneh cites Shabbat 8:3, which states that one is liable for transferring a hide this size from one domain to another. Significantly, in his Commentary on the Mishnah (loc. cit.), the Rambam states that this refers not to the skin used to make parchment on which the amulet is written, but rather the leather used as a covering for the amulet. is liable.17This is also one of the 39 categories of forbidden labor.
Just as one who processes [a hide is liable], so too, is one who salts [a hide], for salting is one of the methods of processing.18See Hilchot Tefillin 1:6. [Prohibitions associated with the forbidden labor of] processing do not apply with regard to foodstuffs.19The Rambam's ruling differs from that of Rabbenu Chanan'el, who holds one liable for salting meat to preserve it for an extended period. (See also the Shulchan Aruch, Orach Chayim 321:2-6, which mentions several Rabbinic prohibitions in connection with salting food. Note the explanation of Shulchan Aruch HaRav 321:2.) The Ma'aseh Rokeach states that the Rambam maintains, by contrast, that there are no Rabbinic prohibitions associated with salting food in this context. Note, however, Chapter 22, Halachah 10, which mentions a Rabbinic prohibition against salting food as part of the pickling process.
Similarly, one who smooths [a portion of an animal's] hide large enough to make an amulet is liable.20This is also one of the 39 categories of forbidden labor. What is meant by smoothing? Removing the hair or the wool from the hide after [the animal's] death21Apparently, the Rambam adds this phrase to distinguish between this category of forbidden labor and the category of shearing. Shearing refers to removing an animal's wool or hair when alive, while smoothing refers to performing the same activity after the animal's death (Yesodei Yeshurun). Note, however, Chapter 9, Halachah 7, which states that one is liable for shearing an animal's wool even when the animal is dead.
See also the Responsum of the Beit Meir (Vol. IV, p. 142), which explains that although the activities included in the two categories of labor are similar, their objectives differ. The object of the labor of shearing is to obtain wool, while the object of the labor of smoothing is to produce a smooth hide.
[According to the Rivosh (cited in Chapter 9, Halachah 7), shearing applies when wool is removed from a live animal, while smoothing applies when the same activity is performed after an animal has died (Eglei Tal).
so that the surface of the hide will be smooth.

הלכה ו
הַמְפָרֵק דּוּכְסוּסְטוּס מֵעַל הַקְּלָף הֲרֵי זֶה תּוֹלֶדֶת מַפְשִׁיט וְחַיָּב. (הַמְפָרֵק מִן הָעוֹר כְּדֵי לַעֲשׂוֹת קָמֵעַ חַיָּב). הַדּוֹרֵס עַל הָעוֹר בְּרַגְלוֹ עַד שֶׁיִּתְקַשֶּׁה אוֹ הַמְרַכְּכוֹ בְּיָדוֹ וּמוֹשְׁכוֹ וּמַשְׁוֶה אוֹתוֹ כְּדֶרֶךְ שֶׁהָרַצְעָנִין עוֹשִׂין הֲרֵי זֶה תּוֹלֶדֶת מְעַבֵּד וְחַיָּב. הַמּוֹרֵט נוֹצָה מִן הָאֶבְרָה הֲרֵי זֶה תּוֹלֶדֶת מוֹחֵק וְחַיָּב. וְכֵן הַמְמָרֵחַ רְטִיָּה כָּל שֶׁהוּא אוֹ שַׁעֲוָה אוֹ זֶפֶת וְכַיּוֹצֵא בָּהֶן מִדְּבָרִים הַמִּתְמָרְחִין עַד שֶׁיַּחֲלִיק פְּנֵיהֶם חַיָּב מִשּׁוּם מוֹחֵק. וְכֵן הַשָּׁף בְּיָדוֹ עַל הָעוֹר הַמָּתוּחַ בֵּין הָעַמּוּדִים חַיָּב מִשּׁוּם מוֹחֵק:
כסף משנה
6.
A person who separates duchsustos from k'laf22See Hilchot Tefillin 1:7-8, where the Rambam explains that a hide used for parchment is divided in half. The thin upper side of the hide where the hair grows is called k'laf. The thick portion on the side of the flesh is called duchsustos. Tefillin should be written on k'laf, while mezuzot should be written on duchsustos.
At present, the parchment used for writing sacred articles is no longer processed in this fashion.
is liable for [performing] a derivative [of the forbidden labor] of skinning. A person who separates [a portion] from a hide large enough to make an amulet is liable.23This phrase is not included in the early manuscripts and printings of the Mishneh Torah. Its addition appears to be a printing error.
A person who treads upon a hide with his feet until it becomes hard, or one who softens it with his hands, extending it, and leveling it as the leather workers do is liable for [performing] a derivative [of the forbidden labor] of processing.24See Chapter 23, Halachah 10, for Rabbinical prohibitions associated with leather working.
A person who pulls a feather from the wing of a fowl is liable for [performing] a derivative [of the forbidden labor] of smoothing. Similarly, a person who smears a poultice of even the smallest size, beeswax,25See Chapter 23, Halachah 11, for Rabbinical prohibitions associated with smoothing wax. tar, or other entities that are smeared until a smooth surface is produced is liable for [performing a derivative of the forbidden labor of] smoothing.
A person who rubs a hide that is suspended between pillars is liable for smoothing.26The Maggid Mishneh cites the Jerusalem Talmud (Shabbat 7:2), which explains that preparation of the hides used for the Sanctuary involved suspending them on pillars and rubbing them so that their surface would be smooth.

הלכה ז
הַמְחַתֵּךְ מִן הָעוֹר כְּדֵי לַעֲשׂוֹת קָמֵעַ חַיָּב. וְהוּא שֶׁיִּתְכַּוֵּן לְמִדַּת אָרְכּוֹ וּמִדַּת רָחְבּוֹ וְיַחְתֹּךְ בְּכַוָּנָה שֶׁהִיא מְלָאכָה. אֲבָל אִם חָתַךְ דֶּרֶךְ הֶפְסֵד אוֹ בְּלֹא כַּוָּנָה לְמִדָּתוֹ אֶלָּא כְּמִתְעַסֵּק אוֹ כִּמְשַׂחֵק הֲרֵי זֶה פָּטוּר. הַקּוֹטֵם אֶת הַכָּנָף הֲרֵי זֶה תּוֹלֶדֶת מְחַתֵּךְ וְחַיָּב. וְכֵן הַמְגָרֵד רָאשֵׁי כְּלוּנְסוֹת שֶׁל אֶרֶז חַיָּב מִשּׁוּם מְחַתֵּךְ. וְכֵן כָּל חֲתִיכָה שֶׁיַּחְתֹּךְ חָרַשׁ עֵץ מִן הָעֵצִים אוֹ חָרַשׁ מַתֶּכֶת מִן הַמַּתָּכוֹת חַיָּב מִשּׁוּם מְחַתֵּךְ. הַנּוֹטֵל קֵיסָם שֶׁל עֵץ מִלְּפָנָיו וּקְטָמוֹ לַחְצֹץ בּוֹ שִׁנָּיו אוֹ לִפְתֹּחַ בּוֹ אֶת הַדֶּלֶת חַיָּב:
כסף משנה
7.
A person who cuts [a portion] from a hide large enough to make an amulet is liable,27This is one of the 39 categories of forbidden labor. provided he cuts with a specific length and width in mind. Cutting in this manner is considered as labor [forbidden on the Sabbath]. If, however, one cuts with a destructive intent,28As explained in Chapter 1, Halachah 17, anyone who performs a forbidden labor for a destructive intent is not liable. or without a precise measure, doing so either without thought entirely29As explained in Chapter 1, Halachah 11, anyone who performs a forbidden labor casually, without intent, is not liable. or for pleasure,30Rav Kapach explains that here also the intent is performing the activity for the sake of tension release, without any concern for what one is doing. he is not liable.
A person who trims [the down from] a wing [of a fowl]31To use in stuffing a pillow or a blanket (Rashi, Shabbat 74b). is liable [for performing] a derivative [of the forbidden labor] of cutting. Similarly, one who planes32Our translation is based on the gloss of the Maggid Mishneh, who contrasts this halachah with Chapter 10, Halachah 16. (See the notes on that halachah.) a beam of cedar wood is liable for cutting. Similarly, anyone who cuts a piece of wood or a piece of metal is liable for cutting.
A person who takes a small piece of wood33As is obvious from the following halachah, this refers to a twig or piece of wood that is not fit to be used as food for an animal. and trims it to use as a toothpick or to pry open a door is liable.34The Maggid Mishneh and others question the Rambam's ruling, noting that according to Beitzah 33a-b, it would appear that one is liable only if one trims a piece of wood with a utensil. Significantly, however, the Magen Avraham 322:4 quotes the Rambam's decision without objection.

הלכה ח
כָּל דָּבָר שֶׁהוּא רָאוּי לְמַאֲכַל בְּהֵמָה כְּגוֹן תֶּבֶן וַעֲשָׂבִים לַחִים וְהוּצִין וְכַיּוֹצֵא בָּהֶן מֻתָּר לִקְטֹם אוֹתָן בְּשַׁבָּת מִפְּנֵי שֶׁאֵין בָּהֶן תִּקּוּן כֵּלִים. וּמֻתָּר לִקְטֹם עֲצֵי בְּשָׂמִים לְהָרִיחַ בָּהֶן אַף עַל פִּי שֶׁהֵן קָשִׁים וִיבֵשִׁין. וּמְפַשֵּׁחַ מֵהֶן כָּל מַה שֶּׁיִּרְצֶה בֵּין שֶׁפָּשַׁח עֵץ גָּדוֹל בֵּין שֶׁפָּשַׁח עֵץ קָטָן:
כסף משנה
8.
Any article that is fit to be used as animal fodder35Although the Rambam's ruling is accepted as halachah by the later authorities, the Sefer Mitzvot Gadol and other Rishonim differ. - e.g., straw, soft grasses, palm branches, and the like, may be trimmed on the Sabbath, because the the concept of preparing a utensil does not apply in this context.
It is permitted to break fragrant branches [by hand] for the sake of their scent36E.g., myrtle branches, which give off their fragrance when they are broken open and rubbed. Significantly, when mentioning this law, the Shulchan Aruch (Orach Chayim 322:5) states that this is permitted "for the sake of a sick person." Nevertheless, the conclusion of the later authorities is that a healthy person may also do so (Mishnah Berurah 322:16). although they are hard and dry. One may strip [their bark] as one desires, regardless of whether one strips [the bark] of a small branch or a large branch.

הלכה ט
הַכּוֹתֵב שְׁתֵּי אוֹתִיּוֹת חַיָּב. הַמּוֹחֵק כְּתָב עַל מְנָת לִכְתֹּב בִּמְקוֹם הַמַּחַק שְׁתֵּי אוֹתִיּוֹת חַיָּב. הכּוֹתֵב אוֹת אַחַת גְּדוֹלָה כִּשְׁתַּיִם פָּטוּר. מָחַק אוֹת אַחַת גְּדוֹלָה וְיֵשׁ בִּמְקוֹמָהּ כְּדֵי לִכְתֹּב שְׁתַּיִם חַיָּב. כָּתַב אוֹת אַחַת וְהִשְׁלִים בָּהּ אֶת הַסֵּפֶר חַיָּב. הַכּוֹתֵב עַל מְנָת לְקַלְקֵל הָעוֹר חַיָּב שֶׁאֵין חִיּוּבוֹ עַל מְקוֹם הַכְּתָב אֶלָּא עַל הַכְּתָב. אֲבָל הַמּוֹחֵק עַל מְנָת לְקַלְקֵל פָּטוּר. נָפְלָה דְּיוֹ עַל גַּבֵּי סֵפֶר וּמָחַק אוֹתָהּ. נָפְלָה שַׁעֲוָה עַל גַּבֵּי הַפִּנְקָס וּמָחַק אוֹתָהּ. אִם יֵשׁ בִּמְקוֹמָהּ כְּדֵי לִכְתֹּב שְׁתֵּי אוֹתִיּוֹת חַיָּב:
כסף משנה
9.
A person who writes37This is one of the 39 categories of forbidden labor. two38One is liable for writing two letters because this resembles the activity necessary for the construction of the Sanctuary. A mark was made on each of the sides of the walls, so that it would be used to match the same walls to each other every time the Sanctuary was erected (Shabbat 12:3, Rashi, Shabbat 75b). letters39The same also applies to a person who writes two numerals (Shulchan Aruch HaRav 340:8). See the notes on the following halachah and on Halachah 17, which discuss the use of marks or symbols. is liable. A person who erases writing so that he can write40One's intent need not be to write on the Sabbath itself. As long as one erases with the intent of writing, one is liable regardless of when one actually writes (Mishnah Berurah 340:13). two letters is liable.41This is also one of the 39 categories of forbidden labor. If a mark was made in error on one of the walls, it was erased so that the correct mark could be made.
A person who writes one large letter the size of two [ordinary] letters is not liable.42For he has only written one letter. One is not liable until one writes two letters, regardless of their size. Nevertheless, writing even a single letter is considered as חצי שיעור (the performance of half the forbidden measure of a prohibition) and forbidden according to the Torah itself (Shulchan Aruch HaRav 340:4, Mishnah Berurah 340:12). In contrast, a person who erases one large letter in a place where two letters can be written is liable.43For his erasure is sufficient to enable two letters to be written.
A person who wrote one letter that concluded a scroll is liable.44In this instance, although only one letter was written, the act is significant, because it completed an entire scroll. There are commentaries on Shabbat 104b, the source for this halachah, who maintain that, in this instance, one is liable for performing the labor מכה בפטיש, completing an article. From the Rambam's wording, however, it appears that he holds him liable for writing.
We see a similar decision with regard to the forbidden labor of weaving. In Chapter 9, Halachah 18, the Rambam writes that one is liable for weaving one thread if by doing so, one completes a garment (Rav Kapach).
A person who writes for the sake of ruining the parchment is liable,45I.e., one writes an ordinary letter on parchment fit to write a Torah scroll (Rav Kapach). for one is liable for the writing itself46Although one has ruined the parchment and thus there is a destructive aspect to one's activity, since the words one wrote are significant, one is liable. and the surface on which [the letters] are written is not significant. If one rubs out writing with the intent of ruining [the writing surface], one is not liable.47For the entire purpose of the labor of erasing is to prepare a writing surface for use.
Should one rub out ink that fell on a scroll or rub out wax that fell on a writing tablet,48In Talmudic times, the word פנקס referred to a book of tablets coated with wax upon which merchants would write with a stylus (Rashi, Shabbat 104b). one is liable49For erasing. With this act, one prepares a writing surface. Note the Sh'vut Ya'akov and others who explain that one is liable for smearing the wax (see Halachah 6). Most authorities, however, accept the Rambam's ruling that one is liable for erasing. if [the rubbed out] portion is large enough for two letters to be written upon it.

הלכה י
הַכּוֹתֵב אוֹת כְּפוּלָה פַּעֲמַיִם וְהוּא שֵׁם אֶחָד כְּמוֹ דָּד תֵּת גַּג רָר שָׂשׂ סָס חָח חַיָּב. וְהַכּוֹתֵב בְּכָל כְּתָב וּבְכָל לָשׁוֹן חַיָּב וַאֲפִלּוּ מִשְּׁנֵי סִימָנִיּוֹת:
כסף משנה
10.
A person who writes the same letter twice and thus produces a word [that has meaning] - e.g., דד תת גג רר שש סס חח50All these pairs of letters have meaning. Hence, one is liable. If, however, one writes a pair of letters that has no meaning, even if it is the beginning of a word - e.g., אא, one is not liable (Maggid Mishneh, based on Shabbat 103b). - is liable.
One is liable for writing in any language and with any characters,51It is necessary for the Rambam to mention both other languages and other characters. Otherwise, one might think that with regard to other languages, one is liable only for writing words from other languages when one transliterates them into Hebrew characters - or conversely, that one is liable for writing with other characters only when writing Hebrew words. With the wording he chose, the Rambam makes it clear that one is liable even when one writes words from another language in the characters of that language.
It must be emphasized that there are authorities who differ with the Rambam's decision. The Ramah (Orach Chayim 306:11) quotes the Or Zarua, who states that one is liable for writing only when one writes in the Assyrian script (i.e., the Hebrew script used for Torah scrolls) or in the classic Greek script. Note, however, the Noda BiY'hudah (Orach Chayim, Vol. II, Responsum 32) and the Be'ur Halachah 306, who refute the Or Zarua's opinion and state that it is not accepted by others. And note the S'dei Chemed (ma'arechet kaf, sec. 111), who brings other opinions in support of the Or Zarua.
or even for making two marks.52In his Commentary on the Mishnah (Shabbat 12:3), the Rambam explains the term "signs" as referring to the use of letters as numbers - e.g., א for one, ב for two.
The Maggid Mishneh interprets these "signs" as referring to symbols that are not letters, but are significant to a reader, such as the reversed nunnim found in the Torah, Numbers 10:35-36. (See also Halachah 17 and Shulchan Aruch HaRav 340:7-8.)

הלכה יא
הַכּוֹתֵב אוֹת אַחַת סָמוּךְ לַכְּתָב אוֹ כְּתָב עַל גַּבֵּי כְּתָב וְהַמִּתְכַּוֵּן לִכְתֹּב חֵי''ת וְכָתַב שְׁנֵי זַיִנִּי''ן וְכֵן כַּיּוֹצֵא בָּזֶה בִּשְׁאָר אוֹתִיּוֹת וְהַכּוֹתֵב אוֹת אַחַת בָּאָרֶץ וְאוֹת אַחַת בַּקּוֹרָה שֶׁהֲרֵי אֵין נֶהֱגִין זֶה עִם זֶה אוֹ שֶׁכָּתַב שְׁתֵּי אוֹתִיּוֹת בִּשְׁנֵי דַּפֵּי פִּנְקָס וְאֵינָן נֶהֱגִין זֶה עִם זֶה פָּטוּר. כְּתָבָן בִּשְׁנֵי כָּתְלֵי זָוִית אוֹ בִּשְׁנֵי דַּפֵּי פִּנְקָס וְהֵן נֶהֱגִין זֶה עִם זֶה חַיָּב:
כסף משנה
11.
Left: Two zeinim in the Assyrian script. Right: A chet in the Assyrian script.
A person is not liable for writing in the following circumstances:
He writes one letter next to writing that existed previously;53Even when the letter he wrote completes a word when combined with the writing that existed previously. Since he has not written two letters on the Sabbath, he is not liable (Rashi, Shabbat 104b).
he writes on top of writing that existed previously;54Superimposing his writing on top of a letter that had been written previously. Although this writing made the existent letters clearer, since nothing essentially new is achieved, the person is not liable (ibid.). (See also the latter portion of Halachah 16 and notes.)
he intended to write a chet and instead wrote two zeinim55In the Assyrian script used for Torah scrolls and the like, a chet resembles two zeinim that are connected by two lines referred to as the ch'totrot. or makes a similar error with regard to other letters;
he writes one letter on the floor [of a house] and one letter on [one of] the beams [of the ceiling], for they are not read as a single unit;56Letters that cannot be read together are considered two separate units, and writing each of them a separate activity. Since the obligation for writing on the Sabbath is for writing two letters, as explained above, one is not liable unless the two letters can be read together.
he writes two letters on two pages of a writing tablet that are not read as a single unit.57In his Commentary on the Mishnah (Shabbat 12:5), the Rambam emphasizes that letters must be on the same line to be able to read as a single unit. (See also Shabbat 104b, which emphasizes that the distance between the two letters can also be a significant factor.)
When a person writes [two letters] in two corners [of the walls of a house] or on two pages of a writing tablet and they can be read as a single unit,58This resembles the letters written on the beams of the Sanctuary, the source for the prohibition against writing on the Sabbath. he is liable.

הלכה יב
לָקַח גְּוִיל וְכַיּוֹצֵא בּוֹ וְכָתַב עָלָיו אוֹת אַחַת בִּמְדִינָה זוֹ וְהָלַךְ בְּאוֹתוֹ הַיּוֹם וְכָתַב אוֹת שְׁנִיָּה בִּמְדִינָה אַחֶרֶת בִּמְגִלָּה אַחֶרֶת חַיָּב. שֶׁבִּזְמַן שֶׁמְּקָרְבָן נֶהֱגִין זֶה עִם זֶה וְאֵינָן מְחֻסָּרִין מַעֲשֶׂה לִקְרִיבָתָן:
כסף משנה
12.
If a person took a parchment or the like and wrote one letter upon it in one city and traveled on that same [Sabbath] day to another city where he wrote another letter on another scroll, he is liable.59The Maggid Mishneh states that this ruling, based on Shabbat 104b, applies even when it is necessary to fold the two parchments so that the two letters can be placed in juxtaposition to each other. [This decision is rendered] because when the [two parchments] are brought close to each other, they can be read as a single unit. All that is necessary is to bring them together.

הלכה יג
הַכּוֹתֵב אוֹת אַחַת אַף עַל פִּי שֶׁקּוֹרִים מִמֶּנָּה תֵּבָה שְׁלֵמָה פָּטוּר. כֵּיצַד. כְּגוֹן שֶׁכָּתַב מ' וְהַכּל קוֹרִין אוֹתָהּ מַעֲשֵׂר. אוֹ שֶׁכְּתָבָהּ בְּמָקוֹם מִנְיָן שֶׁהֲרֵי הִיא כְּמוֹ שֶׁכָּתַב אַרְבָּעִים הֲרֵי זֶה פָּטוּר. הַמַּגִּיהַּ אוֹת אַחַת וְעָשָׂה אוֹתָהּ שְׁתַּיִם כְּגוֹן שֶׁחִלֵּק גַּג הַחֵי''ת וְנַעֲשֵׂית שְׁנֵי זַיִנִּי''ן חַיָּב. וְכֵן כָּל כַּיּוֹצֵא בָּזֶה:
כסף משנה
13.

The dark portion is the letter reish. When the serrated lines are filled in, the letter dalet is formed.
A person who writes merely one letter is not liable even when [that letter] is representative of an entire word. What is implied? One wrote a מ and everyone knows that the intent is the word ma'aser60The word ma'aser means "tithes." This abbreviation was often used to refer to money or produce that was ma'aser sheni, "the second tithe," which could be used only to buy food to be eaten in Jerusalem. The same applies regarding other abbreviations. or one wrote [that letter] in the place where a number is required and thus it is as if one wrote [the word] "forty,61The numerical equivalent of מ according to the accepted principles of gematria, Hebrew numerology." one is not liable.
If one was checking a single letter and divided it, [creating] two [letters], one is liable; for example, one divided the connecting lines of a chet, thus creating two zeinim.62See the notes and diagrams accompanying Halachah 11.
Shulchan Aruch HaRav 340:8 mentions that a person who writes a zayin or any other of the letters שטנ"ז ג"ץ that require taggim, "crowns," without these crowns is not liable. From this halachah, it would appear that the Rambam does not accept this ruling, for the zeinim created when the connecting lines of a chet are divided do not have crowns.
The same applies in all similar situations.63As another possibility, Shabbat 104b mentions erasing the projection in the right corner of a dalet to create a reish.

הלכה יד
הַכּוֹתֵב בִּשְׂמֹאלוֹ אוֹ לְאַחַר יָדוֹ בְּרַגְלוֹ בְּפִיו וּבְמַרְפֵּקוֹ פָּטוּר. אִטֵּר שֶׁכָּתַב בִּימִינוֹ שֶׁהִיא לוֹ כִּשְׂמֹאל כָּל אָדָם פָּטוּר. וְאִם כָּתַב בִּשְׂמֹאלוֹ חַיָּב. וְהַשּׁוֹלֵט בִּשְׁתֵּי יָדָיו בְּשָׁוֶה וְכָתַב בֵּין בִּימִינוֹ בֵּין בִּשְׂמֹאלוֹ חַיָּב. קָטָן אוֹחֵז בְּקֻלְמוֹס וְגָדוֹל אוֹחֵז בְּיָדוֹ וְכוֹתֵב חַיָּב. גָּדוֹל אוֹחֵז בְּקֻלְמוֹס וְקָטָן אוֹחֵז בְּיָדוֹ וְכוֹתֵב פָּטוּר:
כסף משנה
14.
A person who writes with his left hand, with the back of one's hand,64Rashi, Shabbat 104b, interprets this to mean that the person holds a pen in his hand and turns his hand upside down to write.
Based on this source, the expression כלאחר יד is used throughout the Rabbinic literature on the Sabbath laws to mean "in an unusual manner."
with his feet, his mouth, or with his elbow, is not liable.65A person is liable for performing a forbidden labor on the Sabbath only when he does so in an ordinary manner. Although there is a Rabbinic prohibition against performing a forbidden labor in an unusual manner, one is not held liable.
A left-handed person who writes with his right hand - which for him is equivalent to other people's left hand - is not liable. If he writes with his left hand, he is liable. A person who is ambidextrous66The precise translation of the Rambam's wording is "uses both of his hands with equal dexterity." On this basis, there is room for question regarding a left-handed person who writes with his right hand, but less comfortably than he does with his left hand. Is he liable for writing with his right hand on the Sabbath or not? is liable regardless of whether he writes with his right or left hand.
When a child holds the pen and an adult holds his hand and moves it, causing him to write, the adult is liable. When an adult holds a pen and a child holds his hand and moves it, causing him to write, the adult is exempt.

הלכה טו
אֵין הַכּוֹתֵב חַיָּב עַד שֶׁיִּכְתֹּב בְּדָבָר הָרוֹשֵׁם וְעוֹמֵד כְּגוֹן דְּיוֹ וְשָׁחוֹר וְסִקְרָא וְקוֹמוֹס וְקַנְקַנְתּוֹם וְכַיּוֹצֵא בָּהֶם. וְיִכְתֹּב עַל דָּבָר שֶׁמִּתְקַיֵּם הַכְּתָב עָלָיו כְּגוֹן עוֹר וּקְלָף וּנְיָר וְעֵץ וְכַיּוֹצֵא בָּהֶם. אֲבָל הַכּוֹתֵב בְּדָבָר שֶׁאֵין רִשּׁוּמוֹ עוֹמֵד כְּגוֹן מַשְׁקִין וּמֵי פֵּרוֹת. אוֹ שֶׁכָּתַב בִּדְיוֹ וְכַיּוֹצֵא בּוֹ עַל עֲלֵי יְרָקוֹת וְעַל כָּל דָּבָר שֶׁאֵינוֹ עוֹמֵד פָּטוּר. אֵינוֹ חַיָּב עַד שֶׁיִּכְתֹּב בְּדָבָר הָעוֹמֵד עַל דָּבָר הָעוֹמֵד. וְכֵן אֵין הַמּוֹחֵק חַיָּב עַד שֶׁיִּמְחֹק כְּתָב הָעוֹמֵד מֵעַל דָּבָר הָעוֹמֵד:
כסף משנה
15.
A person who writes is not liable until he writes with a substance that leaves a permanent mark67As mentioned in Chapter 9, Halachah 13, "Whenever one performs a labor that does not have a permanent effect on the Sabbath, one is not liable." To apply a principle mentioned in the notes to that halachah, if the writing will remain on the Sabbath itself, even if it will fade afterwards, one is liable.
See also the Be'ur Halachah 340 who quotes the opinion of the Rashba (Shabbat 115b) which explains that permanency in this context refers to writing that will last an ordinary period of time.
- e.g., with ink,68In Hilchot Tefillin 1:4, the Rambam describes the preparation of ink as follows:

One collects the vapor of oils, of tar, of wax, or the like, [causes it to condense,] and kneads it together with sap from a tree and a drop of honey. It is moistened extensively, crushed until it is formed into flat cakes, dried, and then stored.
When one desires to write with it, one soaks [the cakes of ink] in gallnut juice or the like and writes with it. Thus, if one attempts to rub it out, he would be able to.
black tint,69The commentaries associate this with a tint produced from the residue of an oven. vermilion,70In his Commentary on the Mishnah (Shabbat 12:4, Megillah 2:2), the Rambam translates this term into Arabic. Rav Kapach states that the Arabic term he uses refers to red colored clay used for drawing. gum, vitriol,71In his Commentary on the Mishnah (loc. cit.), the Rambam describes these substances, using Arabic terms which Rav Kapach translates as referring to saps from trees that are yellow and green in color. When they are mixed with gallnut juice, they turn black, as the Rambam mentions in Chapter 9, Halachah 14. and the like - on a surface on which the writing will remain preserved - e.g., a skin, parchment, paper, wood,72As mentioned previously, the forbidden labor of writing has its source in the letters written as symbols for the walls of the Sanctuary. and the like.
[In contrast,] a person is not liable if he writes with a substance that does not leave a permanent mark - e.g., beverages or fruit juice - or if he writes with ink and the like on a substance like vegetable leaves where the writing will not be preserved. One is liable only when writing with a substance that leaves a permanent mark on a surface where that mark will be preserved.
Similarly with regard to [the forbidden labor of erasing]: A person who erases is liable only when erasing writing that would leave a permanent mark from a surface where that mark will be preserved.73Unless the writing is written with such substances and on such substances, the person's activity is considered insignificant, for the writing would soon fade in any event.

הלכה טז
הַכּוֹתֵב עַל בְּשָׂרוֹ חַיָּב מִפְּנֵי שֶׁהוּא עוֹר אַף עַל פִּי שֶׁחֲמִימוּת בְּשָׂרוֹ מַעֲבֶרֶת הַכְּתָב לְאַחַר זְמַן הֲרֵי זֶה דּוֹמֶה לִכְתָב שֶׁנִּמְחַק. אֲבָל הַמְשָׂרֵט עַל בְּשָׂרוֹ צוּרַת כְּתָב פָּטוּר. הַקּוֹרֵעַ עַל הָעוֹר כְּתַבְנִית כְּתָב חַיָּב מִשּׁוּם כּוֹתֵב. הָרוֹשֵׁם עַל הָעוֹר כְּתַבְנִית כְּתָב פָּטוּר. הַמַּעֲבִיר דְּיוֹ עַל גַּבֵּי סִקְרָא חַיָּב שְׁתַּיִם אַחַת מִשּׁוּם כּוֹתֵב וְאַחַת מִשּׁוּם מוֹחֵק. הֶעֱבִיר דְּיוֹ עַל גַּבֵּי דְּיוֹ וְסִקְרָא עַל גַּבֵּי סִקְרָא אוֹ סִקְרָא עַל גַּבֵּי דְּיוֹ פָּטוּר:
כסף משנה
16.
A person who writes on his skin is liable, because his flesh is [comparable to an animal] hide.74A surface on which writing would be preserved, as mentioned in the previous halachah. Even though the warmth of his flesh will cause the writing to fade afterwards, this is comparable to writing that was erased.75The writing is considered permanent, because the body's heat is considered as an external force that wipes out writing that, in and of itself, would remain permanently. In contrast, a person who engraves the forms of letters onto his skin is not liable.76Engraving is considered equivalent to writing - as is obvious from the law stated immediately afterwards. Nevertheless, in his Commentary on the Mishnah (Shabbat 12:4), the Rambam writes that the person is exempt, because engraving on human skin is not an ordinary way of writing.
A person who cuts out the form of letters on a hide is liable. In contrast, a person who makes a mark in the shape of letters on a hide is not liable.77From the Be'ur Halachah 340 and others, it appears that this refers to a mark that is made with a stylus or the like that will not remain permanently on the hide. (Note the contrast to Hilchot Gerushin 4:7.)
A person who traces over letters that were written with vermilion with ink is liable for two [transgressions]: one for writing and one for erasing.78Letters written with ink are much more attractive and distinctive than letters written with vermilion. Nevertheless, letters written with vermilion are also significant. Hence, by tracing over the initial writing, one performs two activities: the nullification of the letters written previously (erasing) and the composition of new letters (writing). [In contrast,] a person who traces with ink over letters that were written with ink,79To reinforce the previous writing. In this instance, one is not liable, as stated in Halachah 11. who traces with vermilion over letters that were written with vermilion, or who traces with vermilion over letters that were written with ink, is not liable.

הלכה יז
רוֹשֵׁם תּוֹלֶדֶת כּוֹתֵב הוּא. כֵּיצַד. הָרוֹשֵׁם רְשָׁמִים וְצוּרוֹת בְּכֹתֶל וּבְשָׁשַׁר וְכַיּוֹצֵא בָּהֶן כְּדֶרֶךְ שֶׁהַצַּיָּרִין רוֹשְׁמִים הֲרֵי זֶה חַיָּב מִשּׁוּם כּוֹתֵב. וְכֵן הַמּוֹחֵק אֶת הָרִשּׁוּם לְתַקֵּן הֲרֵי זֶה תּוֹלֶדֶת מוֹחֵק וְחַיָּב. הַמְשַׂרְטֵט כְּדֵי לִכְתֹּב שְׁתֵּי אוֹתִיּוֹת תַּחַת אוֹתוֹ שִׂרְטוּט חַיָּב. חָרָשֵׁי הָעֵצִים שֶׁמַּעֲבִירִין חוּט שֶׁל סִקְרָא עַל גַּבֵּי הַקּוֹרָה כְּדֵי שֶׁיִּנְסֹר בְּשָׁוֶה הֲרֵי זֶה תּוֹלֶדֶת מְשַׂרְטֵט. וְכֵן הַגַּבָּלִים שֶׁעוֹשִׂים כֵּן בַּאֲבָנִים כְּדֵי שֶׁיְּפַצֵּל הָאֶבֶן בְּשָׁוֶה. וְאֶחָד הַמְשַׂרְטֵט בְּצֶבַע אוֹ בְּלֹא צֶבַע הֲרֵי זֶה חַיָּב:
כסף משנה
17.
Making designs is a derivative [of the forbidden labor] of writing. What is implied? A person who makes designs or who creates forms80Note the Be'ur Halachah 340, who questions whether one must make two designs to be held liable (as one is liable only when one writes two letters) or one is liable for making a single design. It is explained that from the Jerusalem Talmud (Shabbat 7:2), it appears that a single design is sufficient. on a wall81Our translation follows the standard printed text of the Mishneh Torah. Significantly, some authoritative manuscripts use the Hebrew כחול, meaning "blue," rather that כותל meaning "wall." According to this version, the halachah would read "One who makes designs and forms with blue, red and other [colors] of the like...." or with red color and the like as artists do is liable [for performing a derivative of] writing.82See also Chapter 10, Halachah 16, and notes in regard to making forms on utensils. Similarly, a person who erases a design for the sake of correcting [it]83Alternatively, one is liable if one erases a design to draw a different design in its place. is liable [for performing] a derivative [of the forbidden labor] of erasing.
A person who rules a line in order to write two letters below that line is liable.84This is one of the 39 categories of labor forbidden on the Sabbath. Carpenters who draw a red line on a beam to enable them to saw evenly perform a derivative of ruling a line.85From this halachah and from the Rambam's Commentary to the Mishnah (Shabbat 7:2), it would appear that the category of labor of ruling lines is associated with writing only. This is somewhat difficult, because writing per se, was not performed in the construction of the Sanctuary. Rashi, Shabbat 75b, states that ruling lines was necessary to cut the hides carefully. According to his opinion, it is possible to say that ruling a line in order to saw in a straight line would be considered as the forbidden labor itself and not merely a derivative. See also Shulchan Aruch HaRav 340:11. Similarly, stonemasons who [make lines] on a stone so that they will cut it evenly [perform a derivative of ruling a line.]
One is liable regardless of whether the line one rules is colored or without color.86As the lines of a Torah scroll are ruled.

זמנים הלכות שבת פרק יא
Zemanim Shabbos Chapter 11