Bach's Wohltemperiertes Klavier (BWC) 

 J. S. Bach Das Wohltemperiertes Klavier volumes I-II are epoch-making cycles. Both volumes contain a prelude and fugue in 12 major and minor keys. These 48 superbly composed compositions justly prove the importance of the question of temperament!

Forkel : "He (J. S. Bach) tuned his harpsichord and his keyboard chord, performing the operation with such expert skill that it never took more than a quarter of an hour. The tuning enabled him to play in any key, and made all twenty-four notes available, so that he could modulate to the more distant notes as easily and naturally as to the nearer ones. .... His modulation in chromatic movements was as easy and continuous as in diatonic movements."

 Did Bach create and use a tuning in which he was able to play and modulate clearly in all keys? Manifique!

 According to the theory of Friedrich Wilhelm Marpurg (21 November 1718 - 22 May 1795), Bach used equal temperament. This idea was widely accepted until Herbert Kelletat (1960) disproved it. 

 In the time that has passed since then, there have been many theories about what tuning system Bach used (Kelletat, Kellner, Billeter, Jobin, Leehman, to name a few.)

I am now absolutely certain that Bach used his own tuning system, which I believe is the temperament I suggested. Why am I so sure? However, I hope you will soon understand why I think so, and agree with me, after you have read the BWC QUOTE temperament theory (it won't be complicated), listened to the demonstration recordings and tried it for yourself!