Parallel motion

Audio: parallel motion (0:06)

parallel motion plays parallels on a rock organ. The parallel motion figure shows the score. The two upper parts, soprano and alto, move in parallel motion in each bar. In the first three bars they move by parallel third, sixth and fourth, all of which are valid motions. In the final three bars, they move by parallel fifth and octave, which are not allowed in functional harmony.

Parallel fifths and parallel octaves are banned in functional harmony.

The ban on parallel fifths and octaves is the most famous ban in music. It is a fundamental feature, if not THE most fundamental feature, of functional harmony. It distinguishes functional harmony from every other approach to harmony bar counterpoint, which also bans them, and is where the rule originated. This chapter explains the reasons for the ban.

The rules for relative motion, the motion of chords in functional harmony, is similar to contrapuntal motion. The preferred motion is contrary motion, followed by oblique motion then, lastly, direct motion, similar direct motion being preferred to parallel direct motion. Parallel thirds and sixths are allowed, parallel fifths and octaves are banned. Parallel fourths are banned between the bass and the upper SAT voices, but allowed between the SAT voices themselves.

The usual reason given for banning parallel fifths and octaves is that they destroy the independence of voices (melodies).

The trouble is that melodies are not independent. Functional harmony and counterpoint contain interacting melodies not independent melodies. Independence of melody is a myth. Melody is more constrained in functional harmony and counterpoint than it is in any other approach to harmony. There are three ways in which melody is so constrained and only one of them involves parallels:

1. Functional harmony and counterpoint aim to produce a melody that is singable, ordered and varied. Rules are set to achieve these aims. For example, large leaps over a fifth are prohibited, a melody should not span a tritone, and so on. The aims are laudable but the rules constrain a melody.
2. Melodies interact to create intervals. Certain intervals are deemed pleasing to the ear, consonant, and are widely used, others are deemed dissonant, such as the tritone, and are to be avoided. The rules of consonance impose a further constraint on melody.
3. The rules relating to relative motion ban certain types of parallel motion, namely parallel fifths and octaves. This acts as the final constraint on melody.

Another argument for banning parallel fifths and octaves is that they destroy contrast. Contrary motion, the big idea of counterpoint, and adopted by functional harmony, is the preferred motion because it offers the greatest contrast between a pair of voices. Similar motion is the least preferred option and parallel motion, a type of similar motion, offers no contrast between a pair of voices, because they both sound like one.

Logically, all parallel motion should be banned, not just parallel fifths and octaves. Yet some forms of parallel motion are allowed. Parallel thirds and sixths are not only allowed but positively encouraged. Parallel fourths, banned between the bass and the upper parts, are allowed between the upper parts. This is most surprising, because the fourth is a dissonant interval whilst the fifth is a consonant interval, and it would make more sense to ban parallel fourths before banning parallel fifths.

The decision to ban parallel fifths and octaves is a deliberate design choice. Parallel fifths and octaves are banned not because they destroy the independence of voices nor because they destroy contrast, they are banned because functional harmony chooses to do so. There is no need for any other reason.