'''''Orgelbüchlein''''' ("Little Organ Book") BWV 599\u2212644 is a collection of 46 chorale preludes for organ written by Johann Sebastian Bach. All but three of them were composed during the period 1708-1717, while Bach was court organist at the ducal court in Weimar. The remaining three, along with a short two-bar fragment, were added in 1726 or later, after Bach's appointment as cantor at the Thomasschule in Leipzig.
The collection was originally planned as a set of 164 chorale preludes spanning the whole liturgical year. The chorale preludes form the first of Bach's masterpieces for organ with a mature compositional style in marked contrast to his previous compositions for the instrument. Although each of them takes a known Lutheran chorale and adds a motivic accompaniment, Bach explored a wide diversity of forms in the ''Orgelbüchlein''. Many of the chorale preludes are short and in four parts, requiring only a single keyboard and pedal, with an unadorned ''cantus firmus''. Others involve two keyboards and pedal: these include several canons, four ornamental four-part preludes, with elaborately decorated chorale lines, and a single chorale prelude in trio sonata form. The ''Orgelbüchlein'' is at the same time a collection of organ music for church services, a treatise on composition, a religious statement and a pedagogical manual.