Bach's famous "48"

These are called fully as "The Well tempered (tuned) Klavier (keyboard)". They consist of preludes & fugues in all 24 keys (12 notes in an Octave, both major & minor keys). There are two sets, thus making a total of 48. (BWV 846-893.) They are meant for aspiring students as well as practising musicians. They are considered as the foundation of Western Classical Music, & the "Bible" of piano players. Beethoven could play these when he was 11!

Prior to these, because of "Just" intonation based on mathematical ratios of the frequencies of the notes, keyboard players had some difficulty in playing in remote keys. Bach used the "equal temperament" of multiplying the frequency of each note by the twelfth root of two to get the next note, thus enabling the keyboard players to play in all keys with ease. By the time all semitones are covered by this, it can be noticed that the tonic frequency has  doubled. (Because the equal tempered notes are fractionally different in frequencies than the "Just" intoned notes based on mathematical notes, some wags called them "notes which are always out of tune!")

Though from the title, it may appear that this work may be an academic, pedantic exercise, Bach's genius transforms the work into a work of infinite mood, emotions & variety.

To end on a humorous note, Hans Von Bulow, no lover of encores, on facing a thunderously applauding audience, said "Ladies & Gentlemen, if you do not stop applauding, I will play Bach's 48 from beginning to end!" It takes 4-5 hours to play these, so the audience laughed but stopped applauding, being fully aware that Von Bulow was capable of playing these entirely from memory!