For centuries, equal temperament didn't catch on because musicians tended not to like it. Even when fretted instruments were invented and lutes and guitars were mostly tuned in equal temperament, they still didn't like it. Most especially, musicians didn't like the fat major thirds of equal temperament, which are way out of tune with nature. They preferred the sweet thirds of meantone temperaments, with all their limitations. For another thing, in meantone each key had an audible personality, from, say, the almost-pure and upstanding C major, suitable to moods of equanimity and celebration, to shadowy C minor, suitable for doubt and despair. Equal temperament leaves every key with exactly the same personality, which was widely felt to be boring. Musicians still preferred, then, the old varieties of what is generically called unequal temperament.
In the late 17th century, tuning geeks came up with a new idea: Let's hair-split all over the keyboard, tweaking this and that in minuscule ways, letting, say, a third be a bit larger in one spot and a bit smaller in another. These kinds of flexible temperaments accomplished several things at once: 1) They made all keys usable; 2) yet they preserved the individual character of keys, because each still had its distinctive collection of intervals; 3) and they tamed the big bad wolf.
Hey, said adherents of this more sophisticated unequal system, this really works well! So they called it well-temperament. One of those adherents was J. S. Bach. He wanted, he said somewhat testily, to write in any damn key he felt like, and he tuned his harpsichord himself to make that possible. When a famous organ tuner who did meantone tuning showed up, Bach would play an A flat major chord on one of his organs with its howling wolf, just to torture the old man.
Bach wrote the preludes and fugues of The Well-Tempered Clavier (clavier meaning any kind of keyboard instrument) not only to show off this improved system but to help make well-temperament mandatory by writing irreplaceable pieces in every key. Anybody who wanted to play from the WTC was pressured to use well-temperament, because many of the pieces sounded sour in any other tuning. (However, heh-heh, there's no precise record of which well-tempered system Bach used.) Here, in a new and gorgeous harpsichord recording of the WTC by Peter Watchorn, is the prelude in Eb minor, a key virtually unheard before Bach, and one that clearly for him (and later for Beethoven) represented piercing sorrow. Watchorn's recording is in the current leading surmise concerning Bach's temperament:
The various kinds of meantone and well-temperament help explain why, in the 18th into 19th centuries, keys had particular emotional associations. Key descriptions of the time sound outlandish, and indeed some were on the loony side, but they were founded on the reality that in unequal temperaments each key had its distinctive color and personality. "Is something gay, brilliant, or martial needed?" wrote one theorist. "Take C, D, E [majors]." Another: "D major … the key of triumph, of Hallelujahs, of war-cries, of victory-rejoicing." All those keys were relatively well in tune on the keyboard. Minor keys were innately less in tune, so darker in sound and import: G minor, for example, is "suited to frenzy, despair, agitation. ... The lament of a noble matron who no longer has her youthful beauty." You want a pretty pastoral piece? You want a relaxing key like F major—the key of Beethoven's Pastoral Symphony:
Two of Beethoven's favorite keys tell us a lot about him. The most famous is C minor, described by one writer of the time as "a tragic key … fit to express grand misadventures, deaths of heroes, and grand but mournful, ominous, and lugubrious actions." That's close to how Beethoven interpreted it in the Third and Fifth Symphonies, and in the Pathètique piano sonata, here played by Andras Schiff on a modern equal-tempered Steinway:
On the other hand, in the prevailing unequal temperaments there was still the presence, or at least the ghost, of the old wolf. Thus, croaked one theorist concerning that key, "Death, grave, putrefaction, judgment, eternity lie in its radius." Beethoven studied the theorists carefully, then did what he wanted. As for the putrefaction of A flat major: baloney. For Beethoven, that key, with its complex and distinctive coloration, suggested feelings in the direction of nobility, devotion, and resignation, as in the second movement of the Pathètique, again by Andras Schiff:
When composers stretched for more harmonic variety and tension in the first decades of the 19th century, as a practical matter the once-despised equal temperament won out over unequal tunings, which withered away during the century. But as professor Duffin exemplifies in How Equal Temperament Ruined Harmony, many tuning geeks today still find that temperament loathsome. Actually, Duffin's book is less rabid than its title sounds, amounting to a plea for playing keyboard music through the early 19th century in period tunings, especially in one called "sixth-comma meantone," which Duffin believes is the tuning Bach had in mind for the Well-Tempered Clavier. His reasoning is of Glenn Beckian deviousness. Some claim to find a cabalistic clue to Bach's intended tuning, close to sixth-comma, in the curlicues at the top of his title page for the WTC:
That idea may be nuts, or not. Bach was into puzzles, numerology, and all kinds of musical cabala, so the nuttiest idea of all about his tuning might well be right. It would figure. Listen to Watchorn in the curlicue tuning—or rather a theory about it—in the C# Major Prelude, one of the world's most happy-making pieces. This is another key out of the pale in older temperaments: