Mirror canon

The mirror canon (also called a canon by contrary motion) is a type of canon which involves the leading voice being played alongside its own inversion (i.e. upside-down). The realisation from the 'closed' (unrealised) form can be effected by placing the page in front of a mirror, thus upside down, and beginning with the already progressing first voice.

The Canon a 2 'Quaerendo invenietis' from Johann Sebastian Bach's A Musical Offering, BWV 1079, is a fine example of the process. In its original closed form the alto clef and an upside-down bass clef indicate both the mirror procedure and the appropriate pitches of the voices for the purpose of realisation.