The Great/German Organ Mass comes from the third part of Bach's Clavierubung which consists of the Four Duets and a grouping of organ pieces; these organ pieces make up the Great Organ Mass. This organ mass consists, first, of a prelude and fugue (BWV 552) with the prelude framing the beginning of the mass and the fugue framing the conclusion. Second, there is a series of 21 Chorale-Preludes between the two frames. Some interesting thoughts about this collection are available here.