J.S. Bach - Prelude and Fugue No 3 in C sharp Major BWV 872 from WTC II

The title of Bach's Well-Tempered Clavier refers to the new system of tuning keyboard instruments that had been developed by, amongst others, the Halberstadt organist Andreas Werckmeister (1645-1706), to make it possible to play in all 24 major and minor keys, something which hitherto had been impossible with "mean-tone" tuning. There were, however, several such "well-tempered" tuning in use at that time, and, contrary to earlier assumptions, there is simply no evidence that Bach wrote his two books of preludes and fugues for our modern "equal temperament", which is but one of several possible tunings.