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,
Major thirds tuning tuning's simplicity and consistency are not shared by standard tuning, whose seventh-chord fingering is discussed at the end of this section. Each major-thirds tuning repeats its open-notes after every two strings, which results in two copies of the three open-strings' notes, each in a different octave. This repetition again simplifies the learning of chords and improvisation. This advantage is not shared by two popular regular-tunings, all-fourths and all-fifths tuning. Chord inversion is especially simple in major-thirds tuning. Chords are inverted simply by raising one or two notes three strings. The raised notes are played with the same finger as the