the Bible, the hymn book and the catechisms: Bach had already set numerous biblical texts in his cantatas and passions; in 1736 he had helped prepare a hymn book with Georg Christian Schemelli; finally in 1739 he set the catechism hymns (see earlier illustration of title page) as organ chorale preludes. has suggested the following features that Clavier-Übung III borrowed from Frescobaldi's Fiori musicali, Bach's personal copy of which was signed "J.S. Bach 1714": Intent. The Fiori were written "mainly to assist organists" with compositions "corresponding to mass and vespers". Plan. The first of the three sets of the Fiori consists of a Toccata [prelude] before the mass, 2 Kyries, 5 Christes, followed by a further 6 Kyries; then a Canzone (after the Epistle), a Ricercare (after the Credo), a Toccata Cromatica (for the Elevation) and finally a Canzona [fugue] (after the post-communion). Polyphony. Frescobaldi's short Kyries and Christes are written in four-part stile antico counterpoint. Many of them have a constantly running cantus firmus or pedal point. Structure. The mutations and combination of themes in fugue BWV 552/2 are closely matched by the closing canzona in the first set and the alternative ricercare in the second set of the Fiori. Similarly, the ostinato bass of the fugue BWV 680 is prefigured by a ricercare fugue with a five-note ostinato bass in the Fiori. According to , Bach had a clear liturgical purpose in his organ compendium, with its cyclic order and plan, clear to the eye if not the ear. Even though the manualiter fugues were written at the time as Book 2 of The Well-Tempered Clavier, only the last fugue BWV 689 has anything in common. Bach's musical plan has a multitude of structures: the organum plenum pieces; the three styles of polyphony, manualiter and trio sonata in the Mass; the pairs in the Catechism, two with cantus firmus in canon, two with pedal cantus firmus, two for full organ); and the free invention in the duets. The fughetta BWV 681 at the centre of Clavier-Übung III plays a structural role similar to the central pieces in the other three parts of Bach's Clavier-Übung, to mark the beginning of the second half of the collection. It is written using the musical motifs of a French overture, as in the first movement of the fourth of Bach's keyboard Partitas BWV 828 (Clavier-Übung I), the first movement of his Overture