This is a simple unison canon for two violins with the royal theme appearing in an independent line below the canonic voices. The upside down clef signals the follower to move in the opposite direction of the leader. Stefan Scheller cites this canon as an example of an acoustical illusion in which the scale may be made to appear to rise when it really does not. But a canon it is, and a strict one at that. The highest voice is an exact transposition of the middle upward a perfect 5th.
Bach yet manages to fashion a full-blown fugue.
Notice the unusual use of the bass clef for the canon's follower. The eighth movement is technically not one canon, but two cleverly spliced together in measure 18. The canonic voices of the second half are the vertical mirror image of their counterparts in the first half. The leader reads the ninth canon in the alto clef, right side up, while the follower reads in bass clef, upside down. Like the one before it, this is a mirror canon .