The Well-Tempered Clavier (German Das Wohltemperierte Klavier) refers to two volumes of preludes and fugues by Johann Sebastian Bach. Though the German word "Klavier" today refers to a piano, the word was a generic term for "keyboard instrument" in Bach's day, and various parts of the work would have been played on either harpsichord, clavichord or organ. Each volume contains a prelude and a fugue in every major and minor key, adding up to 24 pairs total. The order of keys begins with C Major, followed by C Minor, then proceeds up by half-step until the 24th in B Minor.
Bach compiled the first book (BWV 846-869) in 1722, followed by the second (BWV 870-893) in 1742, intending them for the study of keyboard playing. They were not published until the 19th century, but after Bach's death they were well-known by some influential Leipzig-based keyboardists such as C. G. Neefe, who taught the young Beethoven in Bonn (who, according to Neefe, had learned the whole thing by heart by age 12). The Dutch ambassador and musical connoiseur Baron Gottfried van Swieten is credited with bringing the WTC to Vienna for the first time in 1777. Because of Swieten's influence, it was known by Mozart. Later, both Frederic Chopin and Sergei Rachmaninoff claimed to have played a prelude and fugue from it every morning.
The first keyboard cycle to include all keys, the WTC inspired several later composers to compose cycles of preludes (with or without fugues). Chopin's Preludes (Op. 28) are particularly beloved, and Rachmaninoff eventually published 24 preludes (Op. 3/2, Op. 23, Op. 32). Claude Debussy also composed 24 preludes, though not with any systematic key progression. Dmitri Shostakovich was so taken with the idea that he published a set of 24 preludes early in his career (Op. 34), and then wrote a separate set of 24 preludes and fugues (Op. 87) in the 1950's, both sets following Chopin's system of progressing through the circle of fifths rather than Bach's half-steps.
The Well-Tempered Clavier is still played by practically all students of the piano and harpsichord, with most of the world's conservatories and music universities including one prelude and fugue as an audition requirement. Several pianists and harpsichordists have recorded the whole cycle, the earliest being by Edwin Fischer in the 1930s. Other notable recordings and live performances have been made by Sviatoslav Richter, Glenn Gould, Roselyn Tureck, András Schiff, and Friedrich Gulda. Wanda Landowska and Ralph Kirkpatrick have both recorded the WTC on harpsichord.