There is a long tradition in classical music of writing music in sets of pieces that cover all the major and minor keys of the chromatic scale. These sets typically consist of 24 pieces, one for each of the major and minor keys.

The Well-Tempered Clavier, BWV 846–893, consists of two sets of preludes and fugues in all 24 major and minor keys for keyboard by Johann Sebastian Bach. In the composer's time, clavier referred to a variety of stringed keyboard instruments, most typically the harpsichord or clavichord, but not excluding the organ, although it is not a stringed keyboard.