Watch the Stacks of Fourths and Fifths online guitar lesson by Matthieu Brandt from Chord Cookbook

Sus2 and sus4 chords can be played as stacks of fourth intervals, creating an odd floating unresolved sound. Another type of chord that gives you this floating ambivalent texture is the 69 chord. If you look at the top of this chord you’ll see the same odd quality. It’s a stack of fourth intervals.

Each note of that chord is an interval of a fourth above the other. This brings us to a different concept we can use in constructing chords, which we can use as a basis for harmony. Up until now we’ve worked with stacks of major and minor thirds. They gave us triads that we could construct a harmony with. You can do the same with stacks of fourth intervals. You still derive the chords from the scale, but this time you don’t use the formula 1-3-5-7, etc, but 1-4-7-10-13, each time skipping two notes. Depending on what kind of scale you use and the position in the scale, you end up with a perfect fourth interval or an augmented fourth.