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.