Voice-leading in Bach chorales: Awkward intervals
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Why are some intervals considered awkward?
There are a number of intervals in the major and minor scales that are only used rarely in vocal melodies because they are awkward to pitch and therefore quite often end up being out of tune.
What intervals should be avoided and where do they appear?
The tritone (augmented fourth or diminished fifth) spans three whole tones and was long avoided in Western music as a melodic interval; its awkwardness led it to be called diabolus in musica (the devil in music). It occurs between the fourth and seventh degrees of both major and minor scales and between the second and sixth of the minor.:
The augmented second appears between the sixth and seventh notes of the minor scale. As well as being awkward to sing, it has a distinctive Eastern (or folk) feel that Western composers have generally avoided.
It is also best to avoid the other odd interval in the minor scale - the diminished fourth/augmented fifth which appears between the third and the seventh note of the minor scale.
How can these intervals be avoided?
In the bass, check that what you think is a perfect fourth or fifth is not in fact a tritone between scale degrees four and seven. It is easy to eliminate the tritone by changing either the inversions or the chord progression itself. In particular, make sure you avoid the following progressions:
In the inner parts, fourths and fifths are rather large leaps in any case and they should be rewritten to smooth this out as in the example below. As so often with chorale exercises, the tritone is not the only problem in this example - there are also parallel fifths between bass and alto.
These are a common and annoying problem when writing in minor keys. In the bass, progressions between VI and VII should be avoided in any case (see harmonic DOs and DON'Ts). Similarly, you cannot write bass progressions between IVb and Vb, putting one chord in first inversion.
The most common problems with augmented seconds, however, are caused by passing notes. The progression from I to VI shown in the example below works well in both major and minor, but if you try and add a passing note in the minor, you will end up with an augmented second (see example b). There are two solutions:
- the first is simply to miss the passing note out
- the second is to flatten the seventh on the way down as in a melodic minor scale (see second half of example below)
Important note: you cannot flatten the seventh or raise the sixth if the note is part of the chord rather than a passing note - in such cases you need to find a different chord progression