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.
A further step towards perfecting this form was taken by Bach when he made the contrapuntal elements in his music a means of reflecting certain emotional aspects of the words. Pachelbel had not attempted this; he lacked the fervid feeling which would have enabled him thus to enter into his subject. And it is entering into it, and not a mere depicting of it. For, once more be it said, in every vital movement of the world external to us we behold the image of a movement within us; and every such image must react upon us to produce the corresponding emotion in that inner world of feeling.
— Philipp Spitta, 1873, writing about the Orgelbüchlein in Volume I of his biography of Bach
Here Bach has realised the ideal of the chorale prelude. The method is the most simple imaginable and at the same time the most perfect. Nowhere is the Dürer-like character of his musical style so evident as in these small chorale preludes. Simply by the precision and the characteristic quality of each line of the contrapuntal motive he expresses all that has to be said, and so makes clear the relation of the music to the text whose title it bears.
— Albert Schweitzer, Jean-Sebastien Bach, le musicien-poête, 1905