In respect to music history, what is a "well tempered Clavier"?

I'm trying to prepare notes for an exam tomorrow but I don't have any information on this. Thanks...

At that time, there was a difficulty with equal temperament, meaning that it was difficult for musicians to tune the instruments so all were in tune together, mostly to the keyboard. On a keyboard, today we take for granted that Ab is also G#, but they really aren't the same tone. There is a very slight pitch difference. It wasn't until Bach figured out how to temper the keyboard so that it would be in tune with every other instrument, and make tuning uniform. Then, he composed The Well-Tempered Clavier, in which he uses all the keys in tonal music.

EDIT: Thanks, and thumbs up to suhwahaksaeng!

"The Well-Tempered Clavier" is a set of didactic keyboard works written by J.S. Bach. There is one piece in each of the 24 keys.

Go to class next time.

Pampered Parrots' answer is mostly correct.
The well-tempered system came into existence during Bach's lifetime.
Until that time, it was customary to tune a keyboard instrument in such a way that it sounded perfect in one key but awful in any other key.
The well-tempered system was a compromise between all the keys. A well-tempered instrument does not sound perfect in any one key, but it sounds tolerably good in all keys.

Pampered Parrot made just one mistake, though:
The well-tempered system was not Bach's own innovation.
Bach's contribution, rather, was writing the set of 48 preludes and fugues in celebration of this innovation, which he entitled the "Well-Tempered Clavier."
Since there are 12 keys in an octave, there are 24 major and minor keys. (12 x 2 = 24.)
Bach's opus consists of 2 volumes, each containing a prelude and fugue in each major and minor key.
Since 24 x 2, Bach's masterpiece is also known as the "48 Preludes and Fugues."

However, there is some debate over what Bach meant by "Clavier." Those who are loyal to the clavichord say that he meant clavichord. Those who are loyal to the harpsichord say that he meant any keyboard stringed instrument. Sorry, I haven't heard from any unbiased witnesses.

suwahak's answer is well-said. We should all keep in mind that well temperament is NOT the same as equal temperament. Both represent different solutions to the problem of "where to put the comma" around the circle of fifths.

It's not a it it's a the. It's 24 or 48 works by J.S. Bach that gave birth to the major use of modulation in the later classic peroid. Just look it up on wikipedia.