Shabbos 12:4-5

Shabbos 12:4

If a person wrote two letters in a single state of unawareness, he must bring a sin offering. If he wrote with ink, pigment, dye, sap, paint or anything else, on two walls at the angle where they meet or on two adjacent boards of a writing tablet so that the letters are read together, he is liable. If a person writes on his own skin, he is liable. If he scratches two letters into his skin, Rabbi Eliezer says he is liable (because he wrote) and Rabbi Yehoshua says he is exempt (because this is not a way in which people normally write).

Shabbos 12:5

If a person wrote using fruit juice or other liquids, dust, sand, or anything else that won’t last, he is exempt. One is also exempt if he wrote using the back of his hand, his foot, his mouth or his elbow; if he wrote a letter next to an existing letter or if he wrote on top of existing letters; if he intended to write a ches but it came out two zayins (the letter ches is formed of two connected zayins); if he wrote one letter on the ground and another on a beam; or if he wrote on two walls or two boards of a writing tablet that are not adjacent so that they are not read together. If he wrote one letter (with punctuation) as an abbreviation (such as M. for monsieur), Rabbi Yehoshua ben Beseira says that he is liable but the Sages say that he is exempt.