U is for Übung
Übung is the German word for 'practice'. In the Bach context it occurs in the title of collections of keyboard music written between 1726 and 1741, and labelled the 'Clavier-Übung'. By using the term in this way, Bach was emulating his predecessor as Kantor of Leipzig, Johann Kuhnau (1660-1722).
In Bach's catalogue, the Clavier-Übung Part I (1726-31) consists of the six partitas BWV 825-30; Part II (1735) contains the Italian Concerto BWV 971 and the Ouverture in the French style BWV 831; Part III (1739) contains organ music (BWV 669-89 and 802-5): the collection is framed by the 'St Anne' Prelude and Fugue in E flat (BWV 552), and includes 'various preludes on the catechism and other hymns for the organ'; Part IV (1741) is the Goldberg Variations.
In the contect of 'practice', Bach could of be overtly didactic in the preparation and publication of his music. The seminal Orgel-Büchlein collection of chorale preludes was dedicated 'to a beginning organist in how to set a chorale in all kinds of ways, and at the same time to be practised in the study of pedalling.' In terms of learning keyboard technique, there is a distinct pedagogical progression in Bach's music which would have guided Bach's son Wilhelm Friedemann through the Clavier-Büchlein, the Inventions and Sinfonias, the Well-tempered Clavier and the six Organ Sonatas.
Interestingly, the German version of 'Practice makes perfect' is 'Übung macht den Meister' (Practice makes the master). The notion of 'practice' is interesting in the context of a catalogue of free-standing keyboard works of the superb quality represented in the Clavier-Übung; in German the word also has a sense of 'the exercise of ...'; this is consistent with an interpretation of the Clavier-Übung as a body of work representing a summation of Bach's musical and spiritual thinking on the liturgy, and the musical means employed to realise it.