Bach wrote The Well-Tempered Clavier as a showcase for a new tuning system that could play in all twelve major and all twelve minor keys. Up until that point, the various European tuning systems only worked for some keys, not all of them. If you were in or near the key of C, you were usually okay, but as you moved further out on the circle of fifths, things got ugly fast. So this new tuning system that actually sounded good in all the keys was an exciting development.
However… no one knows what tuning system Bach used. All we know is that it wasn’t twelve-tone equal temperament, the one we all use now. There were many systems in circulation at the time that people called “well temperament.” Was Bach using Werckmeister? Kirnberger? Kellner? Some idiosyncratic system of his own invention? No one knows. This video sums up the situation well:
Until this gets resolved, at least technology makes it easy to hear these different systems for yourself. I used Oddsound MTS-ESP to run some of the Well-Tempered Clavier preludes through various historical tuning systems. Here’s what I got:
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