Polychords

A pretty common way to create colorful harmonic structures in compositions is to use so-called Polychords. The principle is pretty simple: stacking chords (usually triads) on top of each other.

There are similar principles with different names: Upper Structure Triads and Bitonality so let's maybe first clear up what's the difference.

Upper structure triads also do stack triads on top of each other, however both chords usually follow a scale. For instance a D major triad played on top of a C major triad would be called an upper structure triad as it fills up scale tones from C lydian. So these chords basically are a chord with usually quite a few harmonic extensions.

Bitonality refers to a harmonic situation where two keys sound at the same time. This CAN be chords but could also be two melodies that source their tone material from two different scales.

Polychords don't neccessarily need to establish two tonalities but can just appear very briefly without implying any tonality. Also, they don't need to share a common scale but can be sourced from quite remote scales and can also be not just triads but 4-note chords or even more.

The interesting thing about polychords as opposed to creating a "dissonant chord from scratch" is that they keep quite a bit of transparency in spite of their often quite dissonant nature. Our ear manages to separate the two chords form each other sonically and therefore make more structural sense out of it than a seemingly random stack of dissonant intervals.

Basically every chord can be stacked on every chord. Depending on which chords you choose the result can be fairly consonant to heavily dissonant.

E.g. stacking an Em triad on top of a C major triad might hardly be perceived as Polychord but rather a Cmaj7 chord while a Db major triad on top of a C major triad might create such a bone shattering dissonance that it is hard to hear the two individual chords.

The most successful polychords however stack pretty simple individual chords on top of each other. The more complex the "two source chords" are, the trickier it becomes to create transparency with them.

Polychords have a huge advantage in orchestration as well. It is much easier for players to find their role and tune in an easily understandable structure like a triad than in a dissonant chord. If you for instance orchestrate one triad in the Horns and the other triad in the Trumpets on top, you can create a wonderful dissonance that is tranparent and every player finds his/her role due to the easy intervals around them. And still you get a wonderful and rich harmonic situation.

For practical use stay tuned for the next parts of my Action Piece Walkthroughs where I used a few polychords.