Music Theory: a system of tuning in which pitch intervals have been slightly altered ("tempered") from the values dictated by simple ratios so that music may be played in any scale.

Well-tempered scales, developed and theorized by the organist Andreas Werckmeister (1645-1706) and others in the late 1600s and 1700s, were a vast improvement over the untempered tunings (just intonation) and meantone tunings used throughout Europe at that time. Under those systems, music sounds harmonious or pleasing for one particular key, but most other keys contain many dissonant notes. Under well-temperament, the base key still sounds quite harmonious, while other keys range from harmonious to -- at least -- acceptable. Many interpret J.S. Bach's Well-Tempered Clavier as an argument for the flexibility and power of this system.

Equal-tempered scales, which drove well-tempered scales to extinction in the early 20th century, are often imprecisely called well-tempered. This is the technical distinction: in well-tempered tuning, keys have color, i.e. different keys sound subtly but audibly different; under equal-temperament, all intervals are exactly equal ratios, and every major key sounds identical.

The pieces in the Well-Tempered Clavier were not (as many are erroneously taught today) distributed amongst the keys to prove their interchangeability, but were rather meant to express the individual character of the keys under the (now unknown) well-tempered tuning Bach used. Today, it is very difficult to hear that work (or any others) performed with a tuning similar to that of the composer's, though the difference is fairly small and probably a worthwhile sacrifice for the accessibility and practicality of the equal-tempered scale.

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