Signature

Signatures tell a viewer a lot about the authenticity and dating of a print. The very earliest prints did not have signatures at all, although by the late fifteenth century many artists indicated their authorship of a print by incorporating a signature or monogram into the matrix design. This kind of composition is called “signed in the plate” or a “plate signature.” While some prints were pencil signed as early as the late eighteenth century, the practice of signing one's work in pencil or ink did not really become common practice until the 1880s. Today, it is customary for original prints to be signed. When a print is described simply as “signed” it should mean that it is signed in pencil, ink or crayon. A plate signature or a stamped signature should be described as such. 

Source: http://www.ifpda.org/content/collecting_pr...