A/P is the designation used for an artist proof, or artist's proof. These prints are made prior to the limited edition series. A/P is usually printed in the lower left hand corner and may be followed by a number.
Before modern printing practices, lithographers hand-pulled prints and the printing plate degraded with each subsequent print. The first productions were considered the most desirable, as they were the finest quality. These were usually given to the artist as payment for signing the lithographs, which would then be sold by the publisher.
Current technology ensures all prints are of the same quality; therefore an artist's proof, or a later number in a limited edition series are identical. This applies to prints of artwork and fine art photography. Artist's proofs are not technically necessary today, however the tradition still holds.
Hand-pulled artist's proofs are considered more valuable than the limited edition series, because they are of better quality and are the property of the artist. Artist's proofs usually number 10 per cent or less of an edition and this also increases the value. The art world is polarised as to whether photographic artist proofs or those made with modern printing techniques are as valuable. Collectors should look for proofs signed by the artist.