"In the four-part writing of diatonic seventh chords the fifth may be omitted, in which case the doubled note in a four-part texture should not be a tendency tone. The most common doubling is the root." (in Harmony text) What is a useful definition for a tendency tone in Harmony theory ?


In diatonic harmony, tendency tones tend to be one of two things:

  1. The leading tone, which has a tendency to resolve up to tonic (especially when in an outer voice), and
  2. Chordal sevenths, which have a tendency to resolve down by step (no matter what voice they're in).

The logic is as follows: if you double a tendency tone, since these tendency tones have a tendency to resolve in a particular way, doubling a tendency tone will result in parallel octaves, which is of course one of the most strident errors in four-part voice leading.

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  • 2
    And the reason for avoiding parallel octaves is that harmonics moving together as a group are (kind of biologically) perceived as belonging to the same voice, sort of like tracked as one single dot on the radar, so if you have moving parallel octaves, you don't have four voices anymore. :) Or like, if everyone sings in unison, there's in a sense only one voice, even if they're in different octaves. (I know you know this, but just to add to the answer, it's good to know that the "rules" are for a reason, they're not magical) – piiperi Reinstate Monica Jun 6 at 8:56
  • interesting rationale , merging the voices , as the 2 become one ... – Thomas Jun 18 at 17:32

In a dominant 7th - tonic progression, the notes that form the tritone in the dom7, the 3rd and 7th of the chord, have a strong tendency to resolve to the root and 3rd of the subsequent tonic. (This desire of a dominant to reach a tonic is the harmonic basis of the whole Common Practice thing.)

We avoid doubling these notes because their active nature would predominate too much. And, as @Richard said, because if both doubled instances resolved 'correctly' it would create parallel octaves.

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