The following is taken from Trevor Stephenson’s essay on recording J.S. Bach’s ‘The Well Tempered Clavier.’ In the following quote, he writes about the forgotten art of choosing a tuning for one’s keyboard, and keyboard temperament in general.
“The Well-Tempered Clavier is the only great—or even near great—piece to include its tuning in its title. Bach put this out front because temperament in the eighteenth century was a volatile issue—as it should be yet today. There are so many interesting and valid ways of tuning a keyboard instrument, and the choice of tuning can have a significant effect upon the outcome of the music. Because of the mathematical laws of nature, it is not possible (nor perhaps is it even desirable) to have all the tonalities in a twelve-keyed system perfectly harmonious. The question is how and where to compromise or temper? In the sixteenth and seventeenth centuries, before Bach’s era, meantone temperament was the norm. A Renaissance invention, meantone tuning involved tempering the fifths in the commonly used keys—C, G, D, A, F, Bb, and Eb—so that the thirds in those keys would be absolutely pure and perfectly harmonious, that is, without any beats. Consequently, in meantone tuning, the thirds in the rarely used keys (rarely used in the Renaissance)—such as E, B, F#, Db, and Ab—became wolf-type intervals, howlingly out of tune, but nevertheless expressive if used sparingly. In the eighteenth century, composers began to desire a wider field of modulation and therefore became interested in accessing—and de-wolfing—the rarely used keys at the bottom of the circle of fifths. But, because of the laws of nature, to do this required compromising the purity of the thirds in the commonly used keys.
Well temperament was this compromise. It is a particularly intricate tuning which permits modulation to all keys and yet preserves subtle but important differences among the keys. Well temperament is structured so that keys with few accidentals—such as C, G, and F major —are calmer and more stable than keys with many accidentals—such as F# B, Db, and Ab major—which are very high-energy and effervescent. In well temperament, if you play the primary thirds in all the keys, ascending chromatically (that is, play the notes C and E together to hear the primary color of C major, and then play the notes C# and E# together to hear the primary color of C# major, and so forth), you will hear dramatic shifts in the color of the sound, or beat-speeds, each time you change tonalities. Well temperament necessitates that the half steps in the chromatic scale, and thus the whole steps as well, are not evenly spaced. The result of this variegation is that no two major scales and no two minor scales have quite the same sound. These subtle variations in tuning produce a wealth of musical possibilities, and this is the playing field for The Well-Tempered Clavier. Finally, well temperament should not be confused with equal temperament, which came into widespread use early in the twentieth century. In equal temperament, the half steps are absolutely even in their spacing, the result of which is that there is no unifying structural presence or variety of sound color among the keys.”
(The full essay can be read here.)