Canon (music)
There are different kinds of canon.  Canons can be described according to distances between the entries of the voices.  If the second voice starts one bar (one measure) after the first voice, this is called a “canon at the bar”.  If it starts after only half a bar, it is called a “canon at the half-bar”.  It is even possible to have very close canons, e.g. “canon at the quaver (eighth note)”.  Olivier Messiaen wrote a 3 part canon at the quaver in his ''' for violin and piano.  The pianist’s right hand (playing chords), his left hand and the violinist are the three parts.