Sterling is 925/1000 parts pure silver and is a legally enforceable standard. Coin is more variable; the purity of metal matching, in theory, that of contemporary currency. Occasionally, during periods of shortage, coins were literally used as metal stock, especially in the colonial era. Because of the multiplicity of coinage in use, it has varied from 835/1000 to 925/1000. It was never an enforceable standard like sterling, but was a means for silversmiths, lacking a national standard of assay, to assure clients of the quality of their silver. By the 1820s, with flat-rolled silver stock readily available, it became an arbitrary benchmark set at 900/1000 and remained so until the British sterling standard was adopted by Gorham, Tiffany, and others in the 1850s.
Up until the Civil War, most hollow-ware was made from melted coins which were made of silver assayed at 900 parts pure silver to every 1,000 parts, 25 parts lower than the sterling standard.