Counterpoint Lession 10: The two part canon and invention

The two part canon and invention

Part I -  Two-part canon
As mentioned in earlier lessons from  Form and analysis, Canon in D by Pachalbel is NOT a canon because it does not follow the structure of canon form.  Canon  follows strict rules of imitation.  It stretches beyond restatement of the theme in a second voice.  The leading voice of a canon is called the dux, and the following voice the comes.  In two voice canon, the intervals of imitation are usually Perfect (unisons, 4ths, 5ths, and octaves). 

Here, is an example of canon in Corelli's Sonata No.7 in d minor.
notice that the dux enters at the octave.

part II - The two-part Invention
J.S. Bach wrote all of his inventions to be used for teaching pieces.  Inventions are written in canon form, with a dux, and a comes as the two voices.  The invention uses sequences to modulate between EPISODES (points where the theme returns in a different tonal center.  A typical invention will have four or five episodes before modulating back to the tonic before the final cadence.

Here is a Bach two-part invention, notice the use of sequences to modulate between episodes.