Originally Posted By: Mark Hayes
But if they hit on a fifth and then the next interval is also a fifth, the effect is going to be that of two lines collapsing into one harmonized line.

Exactly! The same applies to parallel octaves.

There's a good page here that has lots of pictures and - more importantly - audio examples: https://www.schoolofcomposition.com/whats-wrong-with-parallel-fifths/

For a non-counterpoint example of the same phenomena, consider writing for 4 horns in a jazz setting. If you've got a chord progression like:

      C | Am7 | F | G7 | C

You've got a mixture of 3-note chords (C, F) and 4-note chords (Am7, G7). If you double one of the notes on the 3-note chord so there are enough notes to give the 4 horns, you risk getting the same sort of "collapse" when the notes in octaves blend together.

To avoid this, you'll typically write all the chords out as 4-note chords, so the harmony maintains the same 4-note texture throughout:

      C6 | Am7 | Fmaj7 | G7 | Cmaj7

The interval of the 6th in the C chord is neutral, so it doesn't change the harmonic function of the chord. The sound of 6th chords is a bit dated, but that's a different story. wink

-- David Cuny
My virtual singer development blog

Vocal control, you say. Never heard of it. Is that some kind of ProTools thing?