Bach Fugue in Eb major
Exposition starts on the first note of m1 in the sop on the dominant and ends in m7 on the dominant (Bb) quaver in the tenor voice as outlined in the score.
The exposition begins with the subject in the sop on the dominant of Eb major, ending on first semiquaver (also the dominant) in m2 followed by a semiquaver extension in the sop voice (beats 3 and 4 measure2) finishing on first half of the down beat (the first quaver in sop) in m3.
Alto picks up the subject in m3 on the tonic and ends on the tonic ie the first semiquaver (Eb) in m4. Interestingly while following the exact rhythmic pattern of the original subject, I noted that the first beat of semiquavers starts with a different descending interval compared to the original subject introduced by the sop… as already mentioned the sop starts and ends the sentence on the dominant while the alto starts and ends on the tonic, thus changing the interval between the first and second note in the alto’s answer, making this a tonic answer despite all the other intervals remaining consistent with a 224 51142 semitone pattern as per the interval pattern of the sop. There is one other change from the original occurring at the interval between the quaver rest with the sop stepping down by a semitone, while the alto steps down 3, which also backs up my deduction that the answer in the alto is tonal. This follows the “rule” of form for the exposition where voices alternate their entries between the dominant and the tonic.
Sop counter subjects on the up beat of bar 3 as highlighted in orange and ends on the first quaver (G) in m4.
The semi quaver pattern starting on the second semiquaver in beat 3 of m4 through to end of m5 is an extension with the alto voice performing an extended version of the extension mentioned earlier with the sop also contributing.
The tenor enters with the subject/answer in m6 on the dominant (again following the rule of form for an exposition as mentioned earlier) and ends on the dominant - the first semiquaver on beat 3 m7. The counter subject in the alto voice also starts on the dominant, as highlighted and ends on the first quaver in beat 3 also in m7
In this case the tenor’s answer is Real as it is an exact replica of the initial sentence we first see in the sop with exact intervals between each note. Ie the Tenor is transposed exactly and follows the original sop’s semitone interval pattern of 3224511421410122.
The exposition ends at this point (after the tenor finishes the subject).
The subsequent counter subjects in the alto and tenor observation:
When the alto’s counter subject appears it is tonal compared with the first appearance of it in the sop back in m3. While the rhythms are exact there are differences in the intervals. However when the CS appears in the tenor it is an exact replica (note for note and in rhythmic value) of what we saw in the sops CS.
In addition I have observed that when the subject and counter subject play in each voice, they alternate between diminution and augmentation in the rhythm; so when one voice is playing semiquavers, the other voice is playing quavers or crotchets and vice versa.