The Well-Tempered Clavier

The Well-Tempered Clavier (also known as WTC, BWV 846–893) is the title of two collections of musical works for solo keyboard instruments by the Baroque composer Johann Sebastian Bach. The first collection was written in 1722 and the second was written in 1742, but they were not published until 1801, fifty years after Bach died. Although the music was written for the harpsichord, which was popular at the time, today it is usually played and heard on a piano.
Both collections have 24 short preludes and 24 fugues to go along with it. All of them contrast with each other: some of them are easy to learn, while others are hard to follow (because there are many different melodies going on at the same time, which is called counterpoint). What is special about them is that each prelude and fugue is written in a different major or minor key. They appear in the book so that they go up by semitones. That is, the first prelude and fugue is in C major, the second is in C minor, the third in C-sharp major (or D-flat major), the fourth in C-sharp minor, and so on, until the last two preludes and fugues are in B major and B minor, in that order.
Other composers would later write music that is also in all 24 major and minor keys. The best known of these are the Preludes by Frédéric Chopin, but this would also later be done by Charles-Valentin Alkan, Sergei Rachmaninoff, and Dmitri Shostakovich, to name a few.
Some of the music that appears in "The Well-Tempered Clavier" has become famous. For example, the French Romantic composer Charles Gounod wrote a setting of Ave Maria that is based on the melody of the first Prelude in C Major in the "WTC".