(1) I count only 20th century U.S. circulating coins (1901 - 2000). (2) I include the following coin series as limited by the above dates: IHC, Lincoln Cents, V nickels, Buffalo nickels, Jefferson nickels, Barber dimes, Mercury dimes, Roosevelt dimes, Barber quarters, Standing Liberty quarters, Washington quarters, State quarters, Barber half dollars, Liberty Walking half dollars, Franklin half dollars, Kennedy half dollars, Morgan dollars, Peace dollars, Ike dollars, SBA dollars, and Sacagawea dollars. (3) I exclude error coins, date varieties, mint mark varieties. etc. Some key examples includes the three legged Buffalo, 1955 DDO cent, 1960 and 1982 small and large date cents, 1945 Mercury dime micro S, and many date overstrikes. (4) I include metal changes during a year such as the 1982 cent and 1942 war nickel. (5) I include intentional changes to the die such as the 1909 VDB cents, 1913 buffalo nickels, 1917 standing liberty quarters, and 1917 walking liberties. (6) I exclude any coins manufactured only in mint sets, special mint sets, proof sets, or silver proof sets. (7) I excludes all commemorative coins, gold coins, and silver eagles. (8) I exclude mint rarities that weren't really meant for "prime time" including such things as the 1913 V-nickel, 1964 Peace dollar, and the 1943 copper cent. (9) I have included difficult coins to get such as the 1909-S VDB cent, 1916-D Mercury dime, and the 1916 Standing Liberty quarter. (10) I have included the all state quarters from P & D mints from 1999 and 2000.