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- One who writes two letters on shabbat is chayav for the melacha of kotaiv, but if one letter is significant enough that qualifies as well. 
- The letters must have some particular meaning. For example, drawing a random line on a paper would not be considered an act of kosaiv, but nevertheless it is forbidden on a rabbinic level.
- Writing any kind of letter or symbol (that represents some idea) is forbidden under Kotaiv.  There is a dispute how to classify drawing designs. 
- Affixing letters to a surface is also considered Kotaiv. Examples include:
- Attaching letters to a wallpaper,
- Placing magnetic letters to a board
- Arranging edible letters onto a cake.
- Snapping Scrabble letter tiles into fitted groves
- However, if the letters exist on a surface already it’s permissible to arrange them in an order. Examples include:
- Arranging number cards in front of the shul to show congregants the correct page number
- Lining up numbers of a combination lock 
- Playing a game that entails placing numbered tiles adjacent to one another
- It is only forbidden by the torah if the writing is permanent, but even if it is temporary it is forbidden by the rabbis. 
In the Mishkan
- The act of writing was used in the Mishkan to identify the position of each of the kerashim (planks). This was accomplished by inscribing a symbol on each keresh. Alternatively, the act of writing was needed to keep track of money and valuable materials that were being donated for the building of the mishkan.
Forms of writing
- It’s forbidden to write on a table with liquids on his finger. 
- It’s forbidden to write letters in fog or steam. 
- Fingerprinting (dipping one’s finger into ink and then placing it on paper) is forbidden as it forms a meaningful image. 
- One may signal in the air, water or on a dry board in the form of letters if it doesn’t leave any mark. 
- Taking a photograph is forbidden as it draws an image, but one does not have to avoid being in a picture taken by a non-Jew for his own purposes. 
- Regarding walking in an area where there is a surveillance camera, see Electronics on Shabbat.
- One may mark (not in the form a letter) a parchment by pressing one's nail on the parchment because it doesn’t last, yet it’s preferable not to do this because some authorities forbid this.  However, all opinions agree that it’s forbidden to mark a paper as it’s soft and the mark will last. 
- One may fold the page of a book to mark the place even if the crease leaves a lasting mark. 
- If a page in a book ripped where words are written, it’s permissible to place the pieces next to one another in order to read it, if one doesn’t have another copy of that book. 
- If pages were stuck together by glue or some or material (like water) then it depends; if the pages are stuck in a place of letters, it’s forbidden to pull them apart because in doing so one breaks the letters, however, if the pages are stuck in a place of no letters, it’s permissible to pull the pages apart. 
Writing on the Side of Books
- Many poskim hold that it is permitted to open a book on Shabbat even though it has letters or pictures on the side of the pages of the book; if, however, there is another similar book available without letters on the side, one should use that one. Also, it’s preferable not to write on the sides of books in order to avoid this issue. 
Other Practical Examples
- It’s permissible to walk in shoes that have words etched into the soles that form words when walking on dirt or snow. 
- One can be lenient to wipe with tissue that has words not of sanctity in languages other than Hebrew. 
- It’s permissible to use a thermometer for which letters appear if the person has fever and when it’s removed from the person the letters disappear. [A digital thermometer is forbidden because using it completes an electric circuit.] 
- Regarding putting together a jigsaw puzzle on Shabbat, see Games on Shabbat.
- Writing on Shabbat by Rabbi Michael Taubes
- Article on Writing on Shabbat by Rabbi Yirmiyahu Kaganoff
- Article on The Melacha of Writing on Shabbat by Rabbi Josh Flug
- Mishna Shabbat 7:2, Daf 73a. Rashi 73a says that writing is counted among the melachot because in the mishkan they would write on each board of the Mishkan in order to return it to its correct place whenever the Mishkan was reassembled
- Rambam Hilchos Shabbos 11:9.
- The Mishna in Masechet Shabbat 73a writes that the prohibited melacha is for two letters but the gemara 104b says if it is one letter that finishes off the book, you are also obligated. The Avnei Nezer 201 explains that this is because it gives it significance.
- S”A 340:24
- Rambam Shabbat 11:9. Beiur Halacha 340:4 b'mashkin writes that based on Shabbat Yerushalmi 7:2 that if you draw a picture you are liable
- Rambam (Shabbos 11:17) considers Roshem to be a toldah of kosaiv , while Rashi (Shabbos 103 s.v. mishum) explains the opinion of Rabbi Yose differently. Mishna Brurah 340:22 codifies the Rambam.
- Magen Avraham 340:10, Mishna Brurah 340:22 (8)
- Sh”t Igrot Moshe O”C 1:135
- Sh"t Tzitz Eliezer 13:44, Sh"t Bitzel Hachochma vol Siman 78, Shevet HaKehati 2:153, Shalmei Yehuda 3:16 in the name of Rav Elyashiv, Mishnat Halachot 5:48, Chelkat Yaakov 3:150, Divrei Shalom 4:51
- Shulchan Aruch 340:4 quoting the Or Zarua 76 that non-permanent ink is forbidden midirabanan.
- Shabbos 103b
- Avnei Neizer 199:10
- S”A 340:4, Mishna Brurah 340:19
- Mishna Brurah 340:20
- Shemirat Shabbat KeHalachata 41:75, Eretz Tzvi Siman 71,Yeshuot Moshe 1:70, Shuirim Metzuyanim BeHalacha 80:55, Chelkat Yakov 3:25, Nachalat Ezra Hadaya O”C 1:7, Vayeshev Moshe 1:62
- Rama 340:4, Mishna Brurah 340:22, Shar Tzion 340:28
- Mayim Chaim O”C 1: 145, Shemirat Shabbat KeHilchata 16:26, Rivivot Efraim 3:230
- S”A 340:5, Birkei Yosef 340:5 that Or Zaruah and Smak forbid, Beiur Halacha s.v. “Mutar”
- Mishna Brurah 340:25
- Menuchat Ahava 22:25, Shevet Hakehati 1:130 writes that it’s better not to fold the pages even during the week so as not to disrespect the sefer. see also Rivivot Efraim 1:223 (11)
- Mishnat Halachot vol 6 Siman 89, Sh"t Be'er Moshe 6:125, Shemirat Shabbat KeHalichata 28:2, Igrot Moshe Y”D vol 2 Siman 75 s.v. “VeDvar HaDaf”
- Shemirat Shabbat KeHilchata 28:1
- Birkei Yosef 340:5, Leviat Chen 120, Sh"t Tzitz Eliezer 13:44, Vayesh Moshe 1:65, Mishna Berura 340:17.
- The Levush 340:4 writes that it is a Torah violation to open or close a book with words stamped on the sides of the pages. He maintains that opening the book is erasing because the letters are broken, and then closing the book again is writing because the letters are reformed. Magen Avraham 340:6 and Chazon Ish 61:1 are machmir for this Levush.
- The Rama in a teshuva (119), however, is lenient based on the Gemara Shabbat (104b) that says that there is a biblical prohibition if a person writes one letter in Tiveriya and one in Tzipori because it is not considered lacking a significant action to bring them together. The Rama infers that moving letters closer to or further from one another is not considered writing or erasing. Similarly, opening the book doesn’t erase the letters, but merely separates the parts of the letters, and closing the book doesn’t write the letters, but just combines the halves.
- The Avnei Neizer 210:1-3 rejects this proof. He argues that separating two letters isn’t considered erasing because the letters still exist, but splitting letters horizontally is considered erasing because the letters become nonexistent. See Rama (ibid.) and Taz 340:2 for resolutions to this difficulty.
- However, Sh"t Rama 119 and Taz 340:2 disagree saying that bringing existing letters together isn't a melacha and since the book is meant to be open and closed it is like opening and closing a door which is certainly permissible and not an issue of boneh and soter. The Rama's leniency is based on the Gemara Shabbat (104b) that says that there is a biblical prohibition if a person writes one letter in Tiveriya and one in Tzipori because it is not considered lacking a significant action to bring them together. The Rama infers that moving letters closer to or further from one another is not considered writing or erasing. Similarly, opening the book doesn’t erase the letters, but merely separates the parts of the letters, and closing the book doesn’t write the letters, but just combines the halves. The Avnei Neizer 210:1-3 rejects this proof. He argues that separating two letters isn’t considered erasing because the letters still exist, but splitting letters horizontally is considered erasing because the letters become nonexistent. See Rama (ibid.) and Taz 340:2 for resolutions to this difficulty. Based on this Avnei Nezer, see also Sh"t Har Tzvi Melechet Kotev 4 writes that even the Levush himself agrees that separating two complete letters isn't an issue of writing. He is just concerned of joining and separating letter fragments but not separating two whole letters.
- Halacha for Ashkenazim: Mishna Brurah 340:17 comments that the minhag is to follow the Rama, yet if one has another sefer without letters on the side, he should use that one instead to be strict for the Levush. Shemirat Shabbat KeHilchata 28:2 writes that it is preferable not to open a book with words or pictures written across the edge of their leaves and are broken and put back together when the book is opened and closed. He concludes that many authorities permit opening the book in such a case. The problem is best avoided by not writing on the edge of books.
- Halacha for Sephardim: Yalkut Yosef (340:8 Din Kotev BeShabbat) writes that it is permitted according to the strict law, however, initially one should avoid writing words on the side of Sefarim.
- Yabia Omer O”C 5:28, Az Nidaberu 8:21, Beir Sharim 2:67, 3:38:3.
- Sh"t Yabia Omer O”C 5:29, Az Nidberu 1 pg 164#129, Sh"t Rivivot Efraim 1:257 related to throwing paper with words on it in water.
- Sh”t Yechave Daat 4:29, Sh”t Tzitz Eliezer 14:30 concludes that if a regular thermometer is available one shouldn’t use this type of thermometer, Minchat Ahava 22:19, Kinyan Torah 3:39