On a piano you could play octave C moving to G-D. The high C moves up to D and the lower C moves up to G.

X:1
L:1/2
M:
K:C
%%score B
V:B            clef=bass
% 1
[V:B]  [C,C] [G,D]

What would four-part harmony guidelines say about this? And would classical pianists play such a comping in the LH?

  • In the RH I would play a C chord (G-C-E) moving to a G chord (G-B). How would the guidlines deal with this? – Hank Sep 14 at 13:34
  • I don't understand what I did. – Hank Sep 14 at 13:45
  • You probably were editing while I submitted my edit, then overrode it by submitting yours. – leftaroundabout Sep 14 at 13:50

The rule here is most often referred to as "similar" or "hidden" fifths (or whatever the perfect interval might be).

The rule varies depending on who you ask, but it often states that if the soprano and bass move by similar motion to create a perfect fifth or octave, the soprano must move by step. Since your upper voice moves C up to D (a step), this is correct.

If, however, you had this:

X:1
L:1/2
M:
K:C
%%score B
V:B            clef=bass
% 1
[V:B]  [C,C] [A,E]

This would be an error since the soprano leaps into this perfect interval.

In standard four part harmony, that's fine. You want to avoid parallel fifths and parallel octaves, but to move from an octave to a fifth is fine, assuming there are no other voice-leading issues, which in your example, there are none that I can see.

This could be a left-hand comp, provided that the third of the chord is present in the right hand. Without the third, you cannot interpret the chord quality (major or minor).

That being said, it sounds like from your comments that you're playing the thirds of both chords in the RH, so I see no issues with that sort of comp.

In strict two-part counterpoint we are warned against approaching an 8ve or 5th by similar motion. This doesn't apply in 4-part writing.

Classical pianists don't 'comp'. They play what's written.

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