An important and often confused distinction is that between editions of original prints, produced in the same medium as the artist worked (eg etching, or lithography), and reproduction prints (or paintings), which are photographic reproductions of the original work, essentially in the same category as a picture in a book or magazine, though better printed and on better paper. These may be marketed as "limited editions" with investment potential (which is rarely realized), and even signed and numbered by the artist. Some knowledge is often required to tell the difference, and the marketing by the art trade can be deceptive. See special edition for coverage of this issue in various fields.
The aquatints of Goya, which are done in a technique that wears out quickly on the plate, were the first important prints to be published initially in limited editions, which however were not signed or numbered. In fact the plates survived, and since Goya's death several further editions have been published, showing a progressive and drastic decline in quality of the image, despite some rework. Because of this and other cases, "posthumous editions" produced after the death of an artist, and obviously not signed by him, are usually far less sought after. The plates of later prints are often "cancelled" by defacing the image, with a couple of impressions of the cancelled plate taken to document this. This is now expected by collectors and investors, who want the prints they buy to retain their value.
In later times, printmakers recognized the value of limiting the size of an edition and explicitly numbering the prints (e.g., a print numbered 15/30 is the fifteenth print in an edition of 30). The printing of editions with tight controls on the process to limit or eliminate variation in quality has become the norm In monotyping, a technique where only two impressions at most can be taken, prints may be numbered 1/1, or marked "unique". Artists usually print an edition much smaller than the plate allows, for marketing reasons and to keep the edition comfortably within the un-degraded lifespan of the plate; or specific steps may be taken to strengthen the plate, such as electroplating intaglio images, which uses an electric process to put a very thin coat of a stronger metal onto a plate of a weaker metal.
The conventions for numbering prints are well-established, but there are other marks to indicate that the print exists outside of an edition. Artist's proofs are marked "A.P." or "P/A"; monoprints and uniquely hand-altered prints are marked "unique"; prints that are gifted to someone, or are for some reason unsuitable for sale, are marked "H.C." or "H/C", meaning "hors de commerce"--not for sale. The printer is also often allowed to take some impressions for themselves, these are marked with "PP". Finally, a master image may be printed, against which the members of the edition are compared for quality; these are signed-off as "bon à tirer", or "BAT" ("good to print" in French). Sometimes the number of the main, public, edition can be rather misleading - representing 50% or less of the total number of good impressions taken.