Which Bach collection has preludes and fugues in all 24 major and minor keys?

The Well-Tempered Clavier is Bach’s collection of preludes and fugues covering all 24 major and minor keys.

Johann Sebastian Bach created two related books. The first was compiled in 1722, during his period in Köthen, and the second followed about 20 years later, in 1742, after he had moved to Leipzig. Each book contains 24 prelude-and-fugue pairs, so the complete collection is often nicknamed “the 48.”

The keys proceed chromatically: C major, C minor, C-sharp major, C-sharp minor, and so on, ending in B minor. Each prelude is followed by a fugue in the same key. The title refers to a well-tempered tuning system, which made it practical to play in every key without some keys sounding unusably harsh.

The collection is often confused with Bach’s Brandenburg Concertos, the Art of Fugue, or the Goldberg Variations. Those are major Bach works, but none is the complete cycle of 24 paired preludes and fugues in every major and minor key.