The Art of Fugue
In this canon “at the tenth,” the leader and follower first appear at the interval of a tenth. You’ll hear this variant of the AOF subject in the leader:
But when the follower comes in, it is not an octave above the leader as in the Canon alla Ottava, but a tenth above, which puts it into an F major tonality above the D minor of the leader.
Halfway through, however, Bach pulls a lovely fast one; the canon is repeated, but now the leader and follower are separated by an octave. Listen for this change to begin with the re-entry of the canon at 2:08, with the follower at the octave at 2:21.) Thus Bach demonstrates a canon that works at two intervals, a ratcheting up of the technique of double counterpoint to a higher level of complexity.