Polychords describes a method of analysis that allows you to obtain extensions by using two chords of the key. The analysis of chord function, the intervals of it's corresponding mode, and the relationship between each chord to the other chords in a key as extensions of chord voices provides a method of full diatonic comprehension. You can quickly then grasp the entirety of diatonic relationships between chords, mode and Key. For example you can know immediately what chord is produced when a iii7 chord is played with a V7 chord in any key by analyzing the mode and chord relationships. You can also do the same for chords from diatonic variant scales such as the harmonic major or melodic minor. You can also stack chords from different keys to make polychords and quickly calculate their extension from either root.