While what Don Firth says is generally correct, he ruins his argument by stating that (equal temerament) "also made possible such works as J. S. Bach's Twenty-Four Preludes and Fugues for Well-Tempered Clavier. One prelude and one fugue in each key, both major and minor, without having to stop and retune the instrument for each key."

To quote 'Sometimes you will see Bach's term, wohltemperirt, translated as "equal tempered," but that's not what it referred to, and it's not what Bach meant...... Indeed, Bach's Well-Tempered Clavier was not written to demonstrate the superiority of ET, as is often claimed. It wasn't even written for ET, but for an irregular temperament that worked in a wide variety of keys. Such a temperament was convenient, yes, because the player didn't have to retune all the time as the keys changed (as would have been necessary with a regular meantone system), but it's irregularity also meant that the flavor of the chords was slightly different in each key, and the character of each key was slightly different. It is no coincidence that this was the era when descriptions of key characteristics really came into their own.'