Shabbat, Chapter Twelve, Mishnah Five
This mishnah lists cases in which a person is not liable for writing.
1) If he wrote with liquids, or with fruit juice, on the dust of the road, or on writers powder, or with anything that cannot endure, he is exempt.
2) [If he wrote] with the back of his hand, with his foot, with his mouth, or with his elbow;
3) If he wrote one letter near [other] writing,
4) If he wrote on top of other writing;
5) If he intended to write a cheth but wrote two zayyinin;
6) [If he wrote] one [letter] on the ground and another on a beam;
a) If he wrote on two walls of the house,
b) or on two leaves of a ledger which are not read together, he is exempt.
7) If he wrote one letter as an abbreviation:
a) Rabbi Joshua ben Bathyra declares him liable,
b) But the sages exempt him.
Section one: In all of these cases, she is exempt because she didnt write with an ink that will last, or she didnt write on something that will last. Today, this would mean that writing with pencil does not make one liable on Shabbat. Similarly, writing something on the sand on the beach would not make one liable. Again, this doesnt mean that these things are permitted; it only means that one who does so does not incur liability.
Section two: In this case, the person wrote with a part of her body not typically used in writing. She is therefore exempt.
Section three: She is exempt because she didnt write two letters, even though she wrote next to an already existent letter.
Section four: Here, she wrote on top of already existing letters. She is not liable because the letters existed before she wrote them. Even though she may have added to them, she is still exempt.
Section five: Here she intended to write the Hebrew letter chet, but instead of joining the two parts together, she left the top empty and it turned out that she had twice written the letter zayin. Since she did not intend to write two letters, she is exempt.
Section six: In all three of these cases she did not write the two letters next to each other. Therefore she is exempt.
Section seven: In Hebrew sometimes one letter can be an abbreviation for a full word. For instance a resh with an apostrophe can be used to mean Rabbi. According to Rabbi Judah ben Batera, one who writes one letter as an abbreviation is liable. The sages say that she is exempt because she only wrote one letter.