Chords, as you well know, are based on scales, and the most basic form of chord is a triad — a three note chord consisting of the root, 3rd, and 5th of a major scale. The most usual way to play that chord is as a stack of 3rds (the interval between the root and the 3rd of the chord is a major 3rd, and the interval between the 3rd and the 5th is a minor 3rd, hence, a stack of 3rds). But when you invert a chord, you always have a combination of 3rds and 4ths, giving a sense of balance to the chord.

But it is also possible to create a chord using a stack of 4ths by using tones outside the triad, such as 6ths, 7ths and 9ths. In this short video I demonstrate how to create a chord made of the 3rd, 6th, and 9th of the scale which I call a “6/9” chord (but you’re not going to see it in written music, since most people call it a 13th).