To build 7 type chords, we take EON one more step; we go every other note, until we have 4 tones.
There are different ways of understanding 7 chords. We think the most sticky way is consider the type of triad [Major, minor, diminished in a Major key], and the type of 7 [the Major 7 interval is 11 half steps away from the root, while a minor 7 type interval is 10 half steps].
We can also think in terms of types of 3rds [how far each tone is to the next]. Those numbers [5 to the 7] are on the graphic if you would like to integrate that info as well].
We figured out the triads in a Major key in the previous lesson. We will now take EON until we have 4 tones, rather than just 3. When we do this, we get 4 chord qualities from the key: Major 7, minor 7, Dominant 7, and minor 7 flat 5. The m7b5 is also known as half-diminished. I & IV become Major 7's, the V becomes Dominant 7, the ii, iii, and vi become minor 7's, and the vii chord becomes min7b5.
We can see these types of chords written in a number of ways. Our preferences are on the graphic & first in each, but here are some others…