Voice leading rules
Harmonic (chord) progressions are usually written in vocal style, that is, they are written as if the four voices of the mixed choir were to sing the progression. Each part is referred to by the voice name:
This causes a slight problem in that most chord progressions are between triads (chords with three tones) so that we need to add another tone, usually the root of the chord.
The placing of tones in different parts is called the disposition or voicing of the chord. There are two forms of voicing a chord:
- close position, where the distance between the upper three tones is less than an octave;
- open position, where the distance between the upper three tones is greater than an octave.
The bass can take any of the tones in the triad, although the use of the fifth in the bass is restricted to certain cadential formulae.
Melodic motion within parts
Stepwise and leapwise motion are required to achieve a balanced melody. Stepwise motion is by tone or semitone, up or down. Leapwise motion can be by m3, M3, P4, P5, m6, M6 or P8, up or down.
The following intervals are usually avoided: m7, M7 and A4 (d5), although a leap of an A4 (d5) can be used in the soprano when approaching the leading tone.
Melodic motion between parts
Melodic motion between two or more parts is described in one of three ways:
Doublings
- The tone usually doubled is the root of the triad at the octave or double octave, sometimes at the unison.
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- The fifth is sometimes doubled when it is impossible to double the root.
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- The third is doubled according to the tonality of the chord: if major, the third is rarely doubled, if minor or diminished, the third is doubled in preference to the fifth.
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- The leading tone is never doubled, because this leads to parallel octaves, an undesirable effect (compare with the discussion on scales and tendency tones).
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- In major scale harmony, the subdominant of the scale should not be doubled, because this leads to parallel octaves: the resolution is to the third of scale. An exception could be where the subdominant is the chord: the tonic of the chord is in the bass and is doubled by the soprano - the soprano may well move to the mediant, but the bass would move to the tonic of the scale.
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- Tones that are dissonant with the bass (2nds, 4ths, 7ths) should not be doubled.
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- The fifth of the chord may be omitted in the final cadence where the root may be tripled.
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- The third in chord vi following chord V in the major (V-vi) and chord VI following chord V in the minor (V-VI) should be doubled to provide the correct resolution of the leading tone from chord V and to avoid parallel octaves.
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- Parallel octaves are never used in functional harmony, this includes parallel octaves reached by contrary motion.
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- Parallel fifths are never used in functional harmony, this includes parallel fifths reached by contrary motion.
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- Direct octaves and fifths reached by similar motion are limited to the inner voices.
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- Direct octaves may be permitted in the outer voices if the soprano moves by step and the progression is IV-I or V-I.
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- Direct fifths may be permitted in the outer voices if the soprano moves by step .
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