Wikipedia
The Orgelbüchlein ("Little Organ Book") BWV 599−644 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 has a four-fold purpose: it is a collection of organ music for church services, a treatise on composition, a religious statement, and an organ-playing manual.