A typographical error or typo is a mistake mae during, originally, the manual type-setting (typography) of printed material, or more recently, the typing process. The term includes errors due to mechanical failure or slips of the hand or finger, but excludes errors of ignorance. Before the arrival of printing, the "copyist's mistake" was the equivalent for manuscripts. Most typos involve simple duplication, omission, transposition, or substitution of a small number of characters.