[If] one wrote with liquids, with fruit juices, in road dust, in scribes’ dust, or with anything that does not endure, he is exempt. [If one wrote] with the back of his hand, with his foot, with his mouth, or with his elbow; [if] one wrote one letter alongside a written letter; [if] one wrote over [other] writing; [if] one intended to write a ח, but wrote two ז’s; [if one wrote] one [letter] on the ground and one [letter] on a beam; [or if] one wrote on two walls of the house, or on two pages of a ledger, that cannot be read together, he is exempt. [If] one wrote one letter as an abbreviation, R’ Yehoshua ben Beseira holds [him] liable. But the Sages exempt [him].