50 State Quarters to form what is known as a "mule". Only sixteen of these specimens, produced on dollar planchets, escaped from the mint. A 2005 Minnesota double die quarter, as well as a 2005 Minnesota quarter with extra trees (another die error), have both triggered numismatic interest. An unusual die break on some 2005 Kansas quarters created a humpback bison. Relatively more common are Kansas quarters bearing the motto "IN GOD WE RUST." The United States produces proof coinage in circulating base metal and, since 1992, in separately sold sets with the dimes, quarters, and half-dollars in silver. For the silver issues,
Ladder of thirds tonic or central note." (Middleton 1990, p.203) [while pentatonic scales are analyzed as piled fifths] They are "commonplace in post-rock 'n' roll popular music – and also appear in earlier tunes". (Middleton 1990, p.203) Examples include The Beatles' "A Hard Day's Night", Buddy Holly's "Peggy Sue" and The Who's "My Generation", Ben Harney's "You've Been A Good Old Wagon" (1895) and Ben Bernie et al.'s "Sweet Georgia Brown" (1925). Ladder of thirds A ladder of thirds (coined by van der Merwe 1989, adapted from Curt Sachs) is similar to the circle of fifths, though a ladder of thirds differs in