Double canon
Contrapuntal combination of two canons sounding at the same time.
What it means
A double canon combines two independent canons that sound simultaneously, together forming a multi-part, usually four-voice texture. Each of the two canons has its own theme imitated by its own voices, and the two canonic pairs are matched so that they also fit together harmonically.
Musical use
The double canon is among the most demanding forms of strict counterpoint, since the composer must control two simultaneous canonic lines while also keeping them harmonically compatible. It appears mainly in the contrapuntal art music of the Baroque, notably in the works of Johann Sebastian Bach, and later in compositional studies and treatises on strict part-writing.
It should be distinguished from a simple canon, in which only one line of imitation runs, and from a double fugue subject, where two themes appear together but are not necessarily bound by the strict rules of canonic imitation.
Performance practice
When listening to a double canon, first pick out each of the two canonic pairs separately before trying to follow the interaction of all four voices together. Note which voice follows which of the two themes, and mark the entries so you do not lose track of the double imitation while listening or performing.