Chapter 10: The Rules Regarding the Placement of Perfect Consonances

In addition to the rules regarding dissonant and consonant intervals, there are rules concerning the placement of perfect consonances, unisons, octaves and fifths. The reasons behind these rules are as follows: (1) Voices sounding in unison or at an octave and to a lesser extend a fifth, sound very much like the same voice, so that only one voice is perceived where two were written. (2) Octaves and unisons usually occur at cadences; placing them carelessly will make the ear perceive a cadence where none was intended. (3) Counterpoints which overuse the perfect intervals will sound too uniform and will not offer enough variety to the ear. To this end, our forefathers have wisely given us rules for the placement of perfect intervals.

1. Begin and End on Perfect Consonances, that is unisons octaves and fifths. The third may also be used in the final consonance, especially in four or more voice textures.
2. Parallel motion in Perfect Consonances is Forbidden. Two perfect consonances in a row in contrary motion is allowed but discouraged.
3. The same perfect consonance may be repeated above a stationary tenor, but usually not more than three times. This often happens when two voices both repeat the same consonance two or three times.
4. Approach Octaves and Unisons with care. Octaves and unisons generally should not be approached through similar motion (one voice stepping and one voice jumping, both voices stepping is forbidden by rule 2). They may be freely used with oblique motion (one voice stationary the other moving). They should be avoided on the first semibreve of any breve except for a cadence. At cadences, the unison or octave should be approached by contrary motion.

These rules are a matter of style. The restrictions are relaxed and the composer is given more freedom in the case where the composer is writing a fugue. These rules may also be occasionally violated for deliberate effect.

Onward to: Chapter 11: The Rules Regarding the treatment of the Fourth (Diatessaron and Tritone).

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