The Kyrie and the Gloria are the two movements normally included in the Lutheran Missa. Thus, it is likely that Bach completed these two movements at an earlier date and for a different purpose than the other movements of the Mass in B Minor. This setting is very expansive, particularly given that it contains by far the shortest text of any of the movements of the Mass – a mere six words over three lines of text, in an ABA format:

The “easy way out” for Bach would have been to repeat the same music for the second Kyrie that he had used in the first section, but this is not what he does. As I mentioned above, the setting is extensive (both in terms of individual movements and the collectiveKyrie), using different music for each line of text shown above. This likely reflects the attitude of the Lutherans of Bach’s time toward this Ordinary, which had special significance for them – they, who felt unworthy to approach God, and thus pleaded with the Lord for mercy, which is what the Kyrie is all about.

The opening and closing movements are written for chorus, while the middle movement is a duet for two sopranos. The opening of the first movement serves several purposes:

1. its strong opening clearly established the main key of B minor;
2. the strong opening also sounds like a collective cry for mercy from the choir, representing all of God’s people;
3. a lengthy instrumental interlude acts as an overture or prelude, setting the solemn tone of the Kyrie

In measure 30, Bach begins a lengthy fugue, one of many in the Mass, and a form with which Bach is frequently associated. A fugue is a polyphonic form that is both very strict and very free. How is this possible you ask? Simple. The opening section of a fugue, called the exposition, is very strict. The main theme, called the subject, must be introduced in each voice one time before any voice can repeat the subject. But a single voice cannot simply introduce the subject then drop out; part of the complexity of a fugue comes from having to continue parts even whey they don’t have the theme, in a way that they all both sound good together and are musically interesting. (That is, you can’t just hold a pitch for a long time.) Bach introduces the subject first in the tenors, then the altos, soprano I, soprano II, and finally the basses. In a typical fugue, the subject is presented in a different key from the previous entrance; this keeps the fugue interesting: the same melody in the same key over and over would get very boring.

The exposition, therefore, is the strict part. The free part comes after all the voices are in. From here to the end, there are no restrictions or conventions in terms of what the composer must or should do, so long as he alternates passages which contain a complete statement of the subject somewhere with passages which do not (these are called “episodes”). The effect Bach creates here is profound: many different people, all crying “Lord, have mercy” at their own time, in their own way (different keys or accompanying parts); and the somber mood portrayed really gives the feeling that mercy is so dearly needed.

The middle duet might seem surprisingly upbeat, but this provides a break from the heaviness of the choral movements on either side. The duet is scored for violins in unison, continuo, and two solo sopranos, who often sing together in parallel motion (providing a sweet, pleasant sound). this intimate setting might be another attempt to ask God for mercy, though in a more intimate, personal fashion. The active violin part and walking bass line are reminiscent of another Bach piece: the middle movement (#4) of Cantata No. 140, Wachet auf, where the tenors sing a melody, accompanied by a walking bass line and an active unison string part.

The Kyrie closes with another fugue, using a different subject than the first one, though back in the key of B minor.