Music written in all 24 major and minor keys

There is a long tradition in classical music of writing music in sets of pieces that collectively cover all the major and minor keys of the chromatic scale. These sets typically consist of 24 numbers, one for the major and minor key of each key signature. Well known examples include Johann Sebastian Bach's The Well-Tempered Clavier and Frédéric Chopin's 24 Preludes, Op. 28. Such sets are often organized as preludes and fugues, or designated as preludes or études.

Sets that comprise all the enharmonic variants include 30 numbers. Some composers have restricted their sets to cover only the 12 major keys or the 12 minor keys; or only the flat keys (Franz Liszt's Transcendental Études) or the sharp keys (Sergei Lyapunov's Op. 11 set). In yet another type, a single piece may progressively modulate through a set of tonalities, as occurs in Ludwig van Beethoven's Op. 39, two Preludes modulating through all 12 major keys. Some partial sets such as these were intended to complement existing sets, whether written by the same composer or someone else (as in the case of Lyapunov's Op. 11 set of Transcendental Études in the sharp keys, which was designed to complement Liszt's flat-key cycle).

The bulk of works of this type have been written for piano solo, but there also exist sets for piano 4-hands; two pianos; organ; guitar; two guitars; flute; recorder; oboe; violin solo; violin and piano; cello solo; cello and piano; voice and piano; and string quartet. There are examples of attempts to write full sets that, for one reason or another, were never completed (Josef Rheinberger's organ sonatas, Dmitri Shostakovich's string quartets, César Franck's L'Organiste).

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