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 was based on the tower pound, which is of a troy pound.) The "d" in "dwt" stands for "denarius", the ancient Roman coin that equates loosely to a penny. The symbol "d" for penny can be recognized in the notation for British pre-decimal pennies, in which pounds, shillings, and pence were indicated using the symbols "£", "s", and "d", respectively. For example, "£6 11s 8d" indicated six pounds, eleven shillings, and eight pence. Mint weights, also known as "moneyers' weights" were legalised by Act of Parliament dated 17 July 1649 entitled "An Act touching the monies and coins of England".