Kotev is the melakha of expressing ideas precisely using letters, numbers, or meaningful pictures in a manner that allows the idea to last a long time. In the Mishkan, letters were written on the posts that formed its walls so that the order, position, and orientation of each post would always be clear. Sometimes mistakes would be made, so there was a need to erase the letters and replace them with the correct ones. This is the melakha of Moĥek, erasing in order to write other letters (Shabbat 103a; Rashi on Shabbat 73a).
At first glance, Kotev seems to be a trivial and insubstantial melakha that should not have been included on the list of serious melakhot prohibited on Shabbat. However, in truth, writing is the basis for human activity. No matter how smart one was or how good a memory one had, he would still have had difficulty remembering exactly where to place each individual post in the Mishkan. This is the case with all complex matters: if one does not write them down, he will not remember them precisely and he will lose the ability to reconstruct the knowledge he has accumulated and the achievements he has already attained. Through writing, mankind was able to develop scientifically and improve human life.
In order to ensure that information is precise, sometimes it is necessary to erase a mistake in order to replace it with accurate information. Even if a piece of paper has a stain on it and one erases it so he can write letters instead, he transgresses Moĥek, as his erasure prepares the writing surface (SA 340:3).
Sometimes erasing is considered a melakha even if one does not plan to write in the place of the erasure; for example, if there is an extra letter in a Torah scroll which must be erased in order to render the Torah kosher (Shabbat 104b; BHL 340:3, s.v. “ha-moĥek”).
Using a rubber stamp is also considered Kotev. It makes no difference whether the stamp is held with the right or left hand, because the stamp can be easily used with either hand. Similarly, using a printer, photocopier, or fax machine is prohibited by Torah law on Shabbat, since doing so commits meaningful symbols or letters to writing (see MA, OĤ 32:57; Taz, YD 271:8; Igrot Moshe, OĤ 4:40:10).
One may not type letters or characters on a computer or save them to a computer’s memory. It is also forbidden to take photographs, or to record voices or sounds. However, since these actions do not produce stable forms or letters, many maintain that the prohibition is rabbinic.