Voice motion

Two people singing together, each their own line, make more than two melodies: at each note the distance between them changes or stays, and the two go the same way or opposite ways. Comparing two lines sounding at once, a handful of kinds of motion appear (Med 1996, ch. XLIX), and each sounds different.

In the examples below both voices share one staff: the upper notes are the upper voice, the lower notes the lower voice.

Contrary motion

The voices move in opposite directions: one rises while the other falls (Med 1996, ch. XLIX).

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It is the motion that best preserves the independence of the voices. Since they head in opposite directions, the ear never confuses them, and each goes on sounding as a line of its own.

Similar motion

The voices move in the same direction (Med 1996, ch. XLIX), but the distance between them changes. Med calls this direct motion; Blatter calls it similar motion: both rise or both fall, and one covers a larger interval than the other (Blatter 2007, part 4, art. 1).

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Parallel motion

The special case of similar motion: the voices go the same way and keep the same interval between them (Med 1996, ch. XLIX). Parallel thirds:

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One detail of Med's definition: to call the motion parallel, the quality of the interval is not considered (Med 1996, ch. XLIX). The example has both major and minor thirds, and the motion is still parallel, because they are all thirds. In the same way, a perfect fifth followed by a diminished fifth counts as parallel fifths.

Parallel thirds and sixths are the sound of vocal duos, of the harmonised "second voice" of so much popular music. They enrich the line without erasing either part.

Oblique motion

One voice stays put, or repeats its note, while the other moves in any direction (Med 1996, ch. XLIX).

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It is the motion of pedal notes and of common tones held from one chord to the next.

Three motions or four?

Med counts three motions, direct, oblique and contrary, and treats parallel as a kind of direct motion (Med 1996, ch. XLIX). Blatter counts four, contrary, similar, oblique and parallel, and treats parallel as a special case of similar (Blatter 2007, part 4, art. 1).

The difference is only in the count. Both books describe the same situations and agree that parallel motion is similar motion in which the interval does not change. And Blatter points out that the kind of motion can only be judged between two voices at a time: in a four-voice texture, different pairs may be moving in different ways at once (Blatter 2007, part 4, art. 1).

Parallel fifths and octaves

No principle of music theory is as widely known as the rule to avoid parallel fifths and octaves. Yet ever since it appeared in a fourteenth-century treatise, composers have only partly heeded it (Blatter 2007, part 4, art. 1).

Parallel fifths and parallel octaves:

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Med also calls them consecutive, and puts parallel unisons in the same family (Med 1996, ch. XLIX).

Why avoid them

Blatter gives the acoustic reason. When two voices move together an octave apart, the upper one loses its identity to the ear and sounds like a harmonic of the lower one, its second partial. The upper voice disappears as an independent line, and the texture loses a voice (Blatter 2007, part 4, art. 1).

With the fifth the argument is similar: two notes a fifth apart tend to generate a resultant tone an octave below the lower one, and the two voices sound as one. But Blatter adds a second reason, one of style: parallel fifths sound like features of folk and traditional musics rather than of the counterpoint that writing in several voices aims at (Blatter 2007, part 4, art. 1).

Both reasons show the rule's reach. It protects the independence of voices, and it applies where that independence is the goal. Orchestrators and arrangers double melodies and bass lines at the octave all the time, creating parallel octaves on purpose (Blatter 2007, part 4, art. 1): there, the aim is to thicken one line, not to add another.

Hidden fifths and octaves

There is a subtler case. Two voices reaching a fifth by similar motion, from some other interval, make a hidden fifth; if they reach an octave, a hidden octave (Med 1996, ch. XLIX).

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In the first bar a minor sixth moves down by similar motion to a fifth; in the second, a minor sixth moves up to an octave. There are no two fifths or octaves in a row, but the ear, following both lines as they arrive together, almost hears the fifth or octave that was "skipped". That is why strict writing watches these cases too, above all between the outer voices.

One hidden fifth is so common in classical music that it has a name: horn fifths (Med 1996, ch. XLIX).

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Old horns, without valves, could play only the notes of the harmonic series, and two horns moving down together fell naturally into this pattern, third, fifth, sixth. The ear grew so used to it that the hidden fifth, there, sounds like a signature.

What comes next

Voice motions are the vocabulary of counterpoint, the art of writing simultaneous melodies. And between chords they are the stuff of voice leading: which voice stays, which moves, and where to.