The laws of writing mezuzot
1. How one writes the mezuzah. One writes the two paragraphs “Shma” and “Ve-haya im shemoa” in one column on one sheet, and leaves a space at the top as wide as half a fingernail, and a similar half-fingernail space at the bottom. At the leading edge one leaves enough to wrap round it when it is rolled, and at the end, one doesn’t have to leave anything at all beyond that which surrounds the letters (Sha”kh).
2. It must be written in one column, but if it was written in two or three columns, it is kosher. If it was written on two sheets, even if they were sewn together, it is pasul. And it has already been explained in chapter 9 that they must be written sequentially.
3. The space between “Shma” before “Ve-haya im shmoa” – it is a mitzvah to make it setuma. But since it is the custom to start the seventh line with “Ve-haya im shmoa,” we can’t make it setuma, according to most people, except in the opinion of the Rambam.
4. The parchment requires processing lishmah. It is the opinion of most halakhists that if one wrote on parchment that had not been processed lishmah, it is invalid, even post facto. However, if waiting for parchment processed lishmah would prevent someone from doing the mitzvah, one may write on ordinary parchment and fix it without a blessing, and should try to obtain a kosher one.
5. It is the custom to make it 22 lines, with “Al ha-aretz” at the head of the last line; nothing else is written there.
6. All the lines are written the same length, so that not one is longer than its fellow. If a line comes out longer than the line before it, and the next line comes out longer than the one before it, it is kosher. But it must not made tent-shaped – that is, narrow at the top and wide at the bottom, nor may it tail off – that is, wide at the top and narrow at the bottom, nor may it be rounded. Even if only one side was tented or tailed off, it is pasul.
7. One may not add anything else to the inside. On the outside one writes the name Shaddai opposite the space between the paragraphs, and on the mezuzah-case [lit. the reed in which it is put] we make a hole opposite the name, so that it may be seen from the outside. It is also the custom to write on the outside, opposite יקוק אלקינו יקוק, the sequence כוזו במוכסז כוזו, which are the next letters in the aleph-beit. One writes these letters upside-down relative to the letters of the mezuzah, so that each letter is opposite its replacement letter.
8. An individual’s mezuzot should be checked two times in seven years, and public mezuzot (which don’t get so much usage) twice every fifty years (see the Rosh at the end of hilkhot tefillin).