One who writes two letters in one period of forgetfulness is liable. [Whether] he wrote with ink, with orpiment, with vermilion, with gum, with ferrous sulphate, or with anything that marks; on two walls forming a corner, or on two pages of a ledger, that can be read together, he is liable. One who writes on his skin is liable. One who scratches on his skin, R’ Eliezer holds him liable for a sin offering. But R’ Yehoshua exempts [him].