“…if, for instance, in some of the fugal choruses of the Mass in C Minor I cannot follow the part of the fugue clearly, then I feel cheated in a way. I feel that I’m not doing my job.” (Neville Marriner, Choral Journal, August 1985). In the interests of feeling that I’m doing my job ;), below is a brief explication of the “Cum Sancto” fugue.
The fugue subject, as you know, is the long whole note phrase “Cum Sancto Spiritu”, heard first in the basses beginning on the tonic note of C major. It is answered by the tenors beginning on the dominant (the 5th note of the scale). The altos sing the subject in the tonic, the sopranos answer in the dominant, and our first exposition is complete. Here’s a chart of the entire movement:
Exposition I mm. 7-31
Exposition II mm. 35-52. Subject 2x in the tonic and (tenors) in the relative minor (a)
Exposition III mm. 60-67 Sops have subject in F major; basses in tonic
Exposition IV mm. 81-88 Basses in d minor, tenors in a minor
Exposition V mm. 95-101 Altos in E
Exposition VI mm. 105-136 Triumphant return of the subject in the tonic key – basses in canon with the altos, then sopranos and tenors in canon on the dominant. Then basses in canon with the tenors in a minor, followed by altos and sopranos in C. Notice how the melismatic, wildly turning round and round motif is passed around, from tenor, to alto, to soprano, to bass.
Exposition VII mm.138-154 Subject inverted! So nice. The orchestral strings pick up the melismatic figure.
Exposition VIII mm. 167-175 Stretto: each voice entering with the subject 1 measure apart. Notice that Mozart begins with the sopranos in tonic and works down to the basses.
Exposition IX mm. 186 Unison declaration of the subject while the orchestra goes melismatically nuts.
So – those expositions are what we want to hear. Everything else is really fun gravy – or rather, a kind of salsa fresca.
One more note: usually in a fugue there is a countersubject, a contrasting motive. Here there are several contrasting ideas, anchored by the eighth-note rhythms and often following 3 light quarters: