A proof refers to any type of an engraving that is done by the artist for his or her own satisfaction, prior to the artist giving it to the printer for the final product. Some artists work on their proofs for a long time until they are satisfied that they are ready to have the printer take over.
Turner's "Liber Studiorum" is an example of a proof over that the artist labored for a considerable time. A "proof for all letters" is a proof that is given to the printer without the title, lettering and so forth. This type of proof is quite rare and therefore is able to command higher prices than other types. If a proof is one of the first ten or twenty prints that are marked in some way or signed and sold at higher prices, it is called a "Remarque proof".