Voice-leading in Bach chorales: Leading notes
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What's the problem with leading notes?
As its name implies, the leading note (the seventh note of the scale) leads naturally up to the tonic, so other resolutions of this note can sound rather odd. The leading note will most likely crop up in chord V (as chords VII and III are rather more rarely used) which is usually followed by the tonic, so in the example below the leading note resolutions are shown as part of a V to I progression. You can hear that while the resolution up to the tonic (7-8) is the most natural, the leaping down to the fifth (7-5) sounds fine too. The leap to the third (7-3), however, sounds extremely awkward.
What are the rules?
The rule about leading note resolutions is very simple:
You must not resolve the leading note (or the seventh) in a given key to its third.
Where might I break the rules?
The problems most often occur at cadence points. The most common cadence is where the supertonic falls to the tonic (2-1) in the soprano against the roots of V to I in the bass, as in the example below. Because we already have two tonics in the final chord (in soprano and bass) the leading note in this situation can only resolve to 5 unless we triple the tonic (and miss out a note from the chord) or break the 7-3 rule.
You can see in Example B that the resolution from 7-3 is not very good voice-leading in any case, as it leaps more than is ideal for an inner part (see guidelines on leaps).
Notice that the inner parts are swapped round between these two extracts. Swapping inner parts can often make solving a particular voice-leading problem easier.