Lord, y'all don't give old J.S. credit where due. (Or the mesdames Bach either.) Some clarifications:
1. Bach, more than anyone else, gets the credit for the adoption of tempered tuning. While he was not its inventor or its only proponent, his stature was such that when he championed it, and used it to tune organs and other keyboard instruments, it was quickly adopted universally. For those of you not familiar with the subject, tempered tuning is a compromise tuning method which allows an instrument to be played in any key, as compared to natural tuning where an instrument is tuned to a particular key, say C-major, which is fine if you are going to play in that key, or in a closely related key like a-minor or G-maj., but which won't sound right if you try to play in A-flat-maj. or c-sharp min. or some other unrelated key.
2. Bach was the first keyboard player to use the thumbs. Earlier keyboard players used only the fingers, stretched out straight and flat over the keys, which made for some really involved fingerings. Bach not only used the thumbs but also kept his fingers curved over the keys, as is the method drummed into every piano student today.
3. J.S. had 19 children in all, by two wives. The first, Barbara Bach, was the mother of Wilhelm Friedemann and Carl Philip Emmanuel, both notable artists and composers in their own right; she died after eight children. Her husband was away on a trip with his employer when she died; he came home to find she had been buried a week earlier. The second Frau Bach, Anna Magdalena, was made of sterner stuff; she bore 11 children, including Johann Christian, the "London" Bach, who was 16 when his father died. He went to Italy to study and converted to Catholicism, which must have had The Old Wig (as the boys called him) turning over in his staunch Lutheran grave.
4. J.S. Bach's greatest work was "Die Kunst der Fuge", or The Art of Fugue, which he wrote at the end of his life, literally finishing it on his deathbed, if his son Friedemann is to be believed. It is the last word on the Baroque polyphonic style in general and the art of fugue-writing in particular, and, as such, was out of date the day it was done, since polyphonic music, including fugues, was already going out of style in 1750. The work takes a simple subject melody and applies all known treatments, including inversion, retrogression, inversion-retrogression, etc. (see Musick's post above in which he mentions these techniques), also using different time arrangements, switching back and forth from square time to double time to triple time. After a large number of fugues, having exhausted all possibilities of his subject, which is a simple, solemn and rather mournful tune, he introduces, in another voice, another subject, the B-A-C-H motif, which is B-flat-A-C-B in our notation, the only time in his life he ever used the tune composed of the letters of his name. This goes for only a few lines and then the work stops dead in mid-phrase, since, according to Friedemann, it was at that point that he died. He dictated the entire work to his wife and one of his daughters, as he had gone blind and was dying as the result of an infection from an unsuccessful cataract operation (performed without anesthesia, which was unknown in 1750).
