Suppose a song is "in C" and a guitar and bass are playing the notes of C major.
If the singer sings only thirds above the instruments and we isolate the voice, we hear a melody in E Phyrgian:
C D E F G A B Instrumentals: C major sound if C is emphasized E F G A B C D Voice?: E Phyrgian sound!
I doubt this happens often, because:
- vocal melodies are often hummable and the hummed melodies rarely sound Phyrgian
- I'd expect the vocal melody to sound like it's in the same key as the harmony, so the singer probably has to not always sing thirds, but also find a way to emphasize the tonic (at least in general)
Or, if the singer sings the same mod a third above the instruments then most of the notes they sing will clash:
C D E F G A B Instrumentals: C major sound if C is emphasized E F# G# A B C# D# Voice?: E Major sound
So how does choice of set of notes in harmony actually work? I know it's hard to make generalizations, but what are some general ways for the people making music together to do so such that:
- the different voices (such as guitar and vocals) are in different frequency bands
- the different voices sound like they are in the same key
- the different voices have similar modes (probably one won't do minor thirds while the other does major thirds)
- there are pleasing intervals within each voice (for example, both a guitar and a voice each get perfect fourths and fifths in their respective melodies)
I'm sure there are multiple ways of satsifying at least several of these loose constraints: what are some popular solutions?