QUOTE(DaisyChain @ May 25 2007, 08:37 PM)

I have always been lead to believe that "tempered" means "tuned" . Therefore, "The Well Tuned Klavier". Apparently, Bach liked all his students pianos to be in tune, so developed an excersise for each key so that he knew a student could play any one of them in tune.
A lot more complicated than that I'm afraid!!
Well tempered refers to a tuning system which allowed complex chords to be played in any key. Before then instruments had to be tuned to a key, and were very limited in the other keys they could "visit" during a peice.
This is all linked to the mathematics of vibration and, to be honest, I understand it in mathematical terms better than in musical terms.
If you tune a string to (say) C, then tune a second one to the second harmonic of the first (i.e. to G), and a third one to the second harmonic of than one (i.e to D) you will end up with three strings each tuned a 5th apart.
You could play E as the second harmonic of string 3, the third harmonic of string 2 or the fourth harmonic of string 1. However, as the harmonic series is based on a recursive numbers (i.e. like 1 divided by 3), these three verisons of "E" will all be ever so slightly slightly different to each other.
The further into the harmonic series you go, the worse this problem gets until you run up against the "comma of pythagarus" (of triangle fame) where you just end up with discords.
Early music worked around this problem by simply avoinding notes in the harmonic series. Renaisance musicains leared how to cheat by slightly "detuning" each string so that the thirds and sixths weren't too far out of tune with each other which opened up a whole new world of music. "Well tempered" tuning was the first system that allowed ytou to use all the notes so that you could modulate to any key from any other.
Equal temprament is the modern equivalent. In actual fact they actually involve setting the strings ever so slightly out of tune so that you avoid blatent discords.
If you write music that has no harmony, this is not a problem, you don't need equanl temparment and you can tune all you strings (or wahtever) to the natural (pythagorian) harmonic series. This is why eastern music often uses a lot more complex rhythms than western music - their musical cultures avoided the problems involved in harmony and concentrated more on rhythm.
Not well explained I know. Amandal is a string player and a physiscist so she'll do a better job I'm sure. That or do a web search!