ET, Bach and other stuff

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Richard writes: 
> I would have guessed that keys and key signatures along with the
>major and minor scales besides the "modes" would have been known at least
>since the appearance of the keyboard which I have heard can be traced back
>to the 13th century.  The music staff must have had an influence also.

Greetings, 
    The earliest keyboards  dated from at least 200 B.C.  By 1000 A.D. we 
know of organs with several octaves, but no more than 7 notes per.  
Pythagorean tuning is strongly suggested for these instruments by a lot of 
things .    Between the development of the staff, ( ca. 1100, Guido de 
Arezzo) and the mid 1300's when the Halberstadt organ was completed,  the 
number of notes commonly found in the octave grew from the traditional 7 of 
the Pythagorean era to 12, in a form that we still use today. 
    The point can be made that technology was responsible for these increased 
intonational resources.  Not only were musicians now able to record the 
pieces to paper, but the organ's wind pressure became more controlled, 
allowing the practical use of more keys.  Music at this time, in the form of, 
say, Perotin's four part motets,  had moved far beyond Plainchant, Gregorian, 
and Organum form. 
    Walter of Odington writes in the 13th century that English choirs were 
singing pure thirds. This marks a dramatic new use for the interval, which 
had been a harsh discord in the Pythagorean tuning of the previous 15 
centuries.  It is very plausible that the instruments were built to reproduce 
what vocal music was doing, thus, the need for more keys and a change in the 
tuning. 
    From 1200 to 1400 or so,  the tuning world seems to have tried a lot of 
things, from Pythagorean to Just Intonation, but seems to have settled on 
what we call a meantone by the mid 1400's, later quantified by Aaron in 1513. 
 Keyboards took several forms, but the 7/5 12 has always seemed to be the 
most user friendly. 
REgards, 
Ed Foote