Music written in all major or minor keys
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 title page of the first book of J.S. Bach's The Well-Tempered Clavier, which covers all 24 major and minor keys.
Composer Niels Viggo Bentzon wrote 14 complete sets of 24 Preludes and Fugues.
The Well-Tempered Clavier
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.
Title page of Das Wohltemperierte Clavier, Book 1 (autograph)
Bach's autograph of the 4th Fugue of Book 1
Bach's autograph of Fugue No. 17 in A♭ major from the second part of Das Wohltemperierte Clavier
Early version BWV 846a (1720) of the first prelude of the first book, as written down by Bach in his eldest son's notebook