Units of paper quantity size and type of paper being sold. Reams of 500 sheets (20 quires of 25 sheets) were known in England in c1594; in 1706 a ream was defined as 20 quires, either 24 or 25 sheets to the quire. In 18th- and 19th-century Europe, the size of the ream varied widely. In Lombardy a ream of music paper was 450 or 480 sheets; in Britain, Holland and Germany a ream of 480 sheets was common; in the Veneto it was more frequently 500. Some paper manufacturers counted 546 sheets (21 quires of 26 sheets). J.S. Bach's manuscript paper at Weimar
Many-one reduction in: We say that a class C of languages (or a subset of the power set of the natural numbers) is "closed under many-one reducibility" if there exists no reduction from a language in C to a language outside C. If a class is closed under many-one reducibility, then many-one reduction can be used to show that a problem is in C by reducing a problem in C to it. Many-one reductions are valuable because most well-studied complexity classes are closed under some type of many-one reducibility, including P, NP, L, NL, co-NP, PSPACE, EXP, and many others. These classes