All such material, whether just an art sketch or final plate proof, remains an essay until the production and release of a stamp exactly like it. Then it becomes a proof. It should be noted that reprints of stamps previously issued and sold by the Postal Service are not classified as proofs, even though such reprints may have been sold for collectors' benefit only. When an essay was fully approved, one engraver may have done the vignette only and had proofs of his work pulled at any point. Another engraver, completing the frame and lettering, may also have had progress proofs of his work made.
Margins often show the imprint, letters and numbers of the original contract printers.
In the United States these large die proofs usually were printed on India paper and mounted on cards. Those with the engraver's name or an official approval designation are highly valued. Small die proofs have extremely narrow margins, seldom larger than 3mm to 5mm in width. Approximately 300 types were printed for 85 album sets prepared by the Bureau of Engraving and Printing in 1904. These are found on a fibrous, white wove paper.