Bach’s 48 Preludes and Fugues – two sets of 24 Preludes and Fugues, with a major Prelude and Fugue and minor Prelude and Fugue for each note of the chromatic scale.
The set is known as ‘The Well-Tempered Clavier‘ – tempered means tuned, and clavier means keyboard instrument, so that when published, more keyboard musicians would buy copies than specifying it just for harpsichord, or clavichord etc. The first set were composed in 1722, and the second in 1742. We will be exploring preludes and fugues by other composers and the idea of the 24 major/minor keys later this term.
In terms of tuning, it is most likely that Bach was using a tuning of his harpsichord that meant you could play in any key, rather than tuning a harpsichord through perfect fifths related to the key you were performing in.
As an example of this latter tuning, if you’re in C, C, F and G major sound great, but B and D flat majors would sound sour and out of tune to our ears. The ‘well-tuned’ approach (which is, essentially, how we tune keyboards and pianos today) meant those sour moments were avoided by compromising the tuning slightly throughout, and allowed modulations to remote keys without the audience realising.
If you really want to explore the fascinating world of tuning, where A flat isn’t G sharp by another name but occupies an ever-so-slightly-different tuning, this will help:
https://www.youtube.com/watch?v=7GhAuZH6phs
I’ve played an organ in Geneva with split sharp keyboards and pedals – it’s tricky, but it makes a real difference to the tuning!
The first pair – C major and C minor – are especially famous, but here is the F# major prelude and fugue from the first set- a gentle prelude in 12/8 with plenty of imitation between the two parts, and then a three-voice fugue.
The Listening Project Blog 