Keyboard Tuning of 
Johann Sebastian Bach

"Certain people believe that they can find the preceding accord of the equal   
semitones by beginning ut, re, mi, fa etc. on each key of the spinet, or by   
the number of tremblings or beats which the fifth and other tempered         
consonances make: for example, the fifth beats once in each second when it
is tempered as it should be (as much for the organ as for the spinet);         
whereas when it is just it does not beat at all"                                          

- Marin Mersenne (Paris, 1636)