81.3Upper structure vs. polychord

Upper structure

One recognizable chord shape supplies extensions to a more important parent harmony.

Example:

D/C

heard mainly as C Lydian.

Polychord

Two harmonic identities remain perceptually significant.

For example:

C major triad low

and D major triad clearly separated high,

played so each retains its own identity.

Now we may actually hear:

C major and D major.