The number three, the symbol of the Trinity, is at the heart of this volume that contains a total of 27 pieces (3x3x3), 21 chorale preludes (7×3), and three versions of the chorale dedicated to the Trinity (Allein Gott in der Hoh sei Ehr). The key of E-flat major for the prelude and fugue, extremely rare in Baroque organ works, contains three flats. The prelude is based on three contrasting motives and the triple fugue is constructed in three sections and has three subjects.
From: Johann Sebastian Bach (1685-1750): Complete Organ Works (Ricercar Records, catalog number RIC 289, released in 2009). Booklet note by Bernard Foccroulle, with Jerome Lejeune and William Hekkers, p. 76.