If one writes with a fluid, with fruit juice, with road dust, or with writer`s powder, or with anything that cannot endure, he is not culpable. [If one writes] with the back of his hand, with his foot, with his mouth, or with his elbow; if one writes one letter near [other] writing, or if one writes upon writing; if one intends writing a Heth but writes two zayyinin; one [letter] on the ground and another on a beam; if one writes on two walls of the house, or on two leaves of a ledger which are not to be read together, he is not culpable. If one writes one letter as an abbreviation, R. Joshua b. Bathyra [T2] holds him liable, whilst the Sages exempt him.
(Purchase a printed Mishnah)