At Mrs. Green's, a health food grocery store, I picked up a couple of brochures on local Tai Chi classes. One brochure, for classes taught by physicist Robert Chuckrow, mentions a web site. On his web site there is a link to an essay, Historical Tuning, which I found quite interesting:

Few musicians know that keyboard instruments are not tuned today as in BachÅfs day and before. Even fewer people understand the differences and are able to tune a piano or harpsichord as did Bach and other composers before him. Such music was composed to be played on specific tunings such as meantone or well temperament, not equal temperament, which is almost exclusively used today.

Each tuning involves specific frequency ratios of intervals such as fifths, fourths, and major and minor thirds. These specific frequency ratios differ from one way of tuning to another. Subtle differences in these ratios determine how the music sounds. Therefore, an important dimension of music is lost when it is played on a tuning not intended by the composer. This deficiency has certainly been the case for the past century or so, during which time BachÅfs keyboard music, much of which was written for well temperament, has been mostly played on equal temperament.