Coins of the pound sterling also became known as a "pound". (240 actual pennies, however, weighed only 5400 troy grains, known as tower pound, a unit used only by mints. The tower pound was abolished in the 16th century.) The silver penny remained the primary unit of coinage for about 500 years. The purity of 92.5% silver (i.e., sterling silver) was instituted by Henry II in 1158 with the "Tealby Penny" — a hammered coin. Over the years, the penny was gradually debased until by the 16th century it contained about a third the silver content of a proper troy 24 grain pennyweight. By 1915,
Troy weight system. The origin of the troy weight system is unknown. Although the name probably comes from the Champagne fairs at Troyes, in northeastern France, the units themselves may be of more northern origin. English troy weights were nearly identical to the troy weight system of Bremen. (The Bremen troy ounce had a mass of 480.8 British Imperial grains.) An alternative suggestion is that the weights come from the Muslim domains by way of the Gold Dirhem (47.966 British Imperial grains), in the manner that King Offa's weights were derived from the silver Dirhem (about 45.0 British grains). According to Watson,