Johann Sebastian Bach
| BWV | 866 |
| Title | Prelude and Fugue 21 |
| Composed | 1722, Weimar |
| Key | B-flat Major |
| Scoring | Piano or Harpsichord |
| Movements | Prelude Fugue |
| Category | The well tempered Clavier Part 1 |
| Manuscript | Gemeente Museum, Den Haag; State and University library, Hamburg; High School for Music, Berlin; Conservatoire Royale, Brussels; British Museum, London; Music library, Leipzig; Johann Sebastian Bach Institute, Göttingen; Biblioteca Martini, Bologna; Bach Archive, Leipzig; Yale University, New Haven; Society of Music Friends, Vienna; Abbey, Göttweig; National museum, Prague; Prussian State library, Berlin; Fitzwilliam Museum, Cambridge |