CANON, in the ancient music, is a rule or method of determining the intervals of notes.
Ptolemy, rejecting the Aristoxenian way of measuring the intervals in music, by the magnitude of a tone (which was supposed to be formed by the difference between a diapente and a diatessaron), thought that musical intervals should be distinguished, according to the ratios or proportions which the sounds terminating those intervals bear to one another, when considered according to their degree of acuteness or gravity; which, before Aristoxenus, was the old Pythagorean way. He therefore made the diapason consist in a double ratio; the diapente, in a sesquialterate; the diatessaron, in a sesquitertian; and the tone itself, in a sesquioctave; and all the other intervals, according to the proportion of the sounds that terminate them: wherefore taking the canon (as it is called) for a determinate line of any length, he shows how this canon is to be cut accordingly, so that it may represent the respective intervals: and this method answers exactly to experiment, in the different lengths of musical chords. From this canon, Ptolemy and his followers have been called Canonici; as those of Aristoxenus were called Musici.