Bach's most famous fugues are those for the harpsichord in "The Well-Tempered Clavier", which many composers and theorists look at as the greatest model of fugue. "The Well-Tempered Clavier" comprises two volumes written in different times of Bach's life, each comprising 24 prelude and fugue pairs, one for each major and minor key. Bach is also known for his organ fugues, which are usually preceded by a prelude or toccata. "The Art of Fugue", BWV 1080, is a collection of fugues (and four canons) on a single theme that is gradually transformed as the cycle progresses. Bach also wrote smaller single fugues and put fugal sections or movements into many of his more general works. J.S. Bach's influence extended forward through his son C.P.E. Bach and through the theorist Friedrich Wilhelm Marpurg (1718–1795) whose "Abhandlung von der Fuge" ("Treatise on the fugue", 1753) was largely based on J.S. Bach's work.