Johann Sebastian Bach - The Well-Tempered Clavier Book 1

Each of the two volumes of the Well-tempered Clavier contains one prelude and one fugue in every major and minor key. Often called “the 48”, or the “Old Testament” of piano music (the new being Beethoven’s sonatas) it is perhaps the most important keyboard work of all time, and something that every pianist and every composer must tackle in one way or another – Chopin’s obvious allusion to the C major prelude BWV 846 in his Study Op. 10 no 1 is but one illustration of this.
The preludes are very varied in style and often deal with a specific technical feature, while the fugues are remarkable for their wide range of contrapuntal techniques and modes of expression.
The title suggests that Bach intended a well-tempered tuning system which gives a single instrument the ability to play in all 24 keys.
The opposing system in Bach´s day was meantone temperament in which keys with many accidentals sound out of tune.