By using the triads we have just discovered, we can build seventh chords on top of them. Let's see how they all work!
Just like building triads, we stack thirds on top of one another to be able to build various qualities of seventh chords.
First, let's take the base of a major triad and build seventh chords upon it. Let's use a C Major triad for now.
Reminder, a major chord is built by combining a major 3rd and a minor third above it. To make this triad into a Major 7th chord (MM or M7), we add a major 3rd on top. To make the triad a Major-minor 7th chord (Mm or dominant-7th), we add a minor 3rd on top.
The breakdown of 3rds within the above seventh chords:
What happens if the base triad is minor?
This minor triad can add a minor 3rd on top to create a minor 7th chord: mMm
The last type of commonly used 7th chords contain the diminished triad base.
This base also can add a major or minor triad on top.
Here is the diminished triad with a major 3rd on top. We call this a half-diminished seventh chord (ø7, dm, or diminished-minor).
If we add a minor triad to a diminished triad base, we have a fully-diminished seventh chord (°7, dd, or diminished-diminished)
🦜 Polly wants a progress tracker: Can you spell a gø7 chord (G half-diminished 7th chord)?