Artist's proofs are not included in the count of a limited edition, and sometimes the number of artist's proofs, which belong to the artist, can be surprisingly high at twenty or more. By convention, the artist is not supposed to sell these at once.
In old master prints, a proof will be described as belonging to a different state of the print. States are numbered I, II, III etc, in the sequence they were produced, which can normally be determined.
For example Rembrandt, who had his own printing-press and printed at least the earliest copies of his etchings himself, was a great taker of proofs, but these are normally described as different states - his record was nine for one etching, although this involved returning to an older print he had sold many copies of, and largely reworking it.
But the term proof can be used in the case of a clear working proof of an old master print, like the two impressions of Albrecht Dürer's Adam and Eve (1504, British Museum and Albertina, Vienna) which show the figures largely finished but the background with only the main outlines lightly done.
Art historians, curators, and collectors view working proofs as an especially desirable because of their rarity, and the insight they may give into the progress of the work, and because they may well have belonged to the artist. Especially in the case of dead artists, they can be the only evidence of the artist's incremental development of an image, something not usually available with drawings, paintings, or sculpture.
Collectors also usually prefer final artist's proofs even when they are identical to the main edition; if nothing else the print may have been presented to a friend by the artist. Prints are generally sold as limited editions, with a print being cheaper than a drawing or painting because the artist/gallery makes more money by selling multiples. An artist's proof has special value because of its extra rarity and its possible differences from the "standard" print, factors that are often reflected in its price.
Since printmaking is a very technical area, and many processes require expensive equipment, most artists choose to work with specialist printers. The print shop provides technicians skilled in the process; the artist provides the art. It is customary in these cases to pay the technicians with a signed artist's proof, in addition to his wages. The print shop will also retain a proof, normally signed off as "bon à tirer" ("good for printing" in French) to use as a control example against which the other impressions are compared. This has resulted in some very impressive collections of prints owned by printmakers themselves.
An artists proof is also known as a "comp" or "complimentary" if given to printers.