Seventh chords are made by stacking three 3rds. The four notes are called the root, third, fifth, and seventh. The root is the foundational note. The third is a 3rd above the root. The fifth is a 3rd above the third (or a 5th above the root). The seventh is a 3rd above the fifth (or a 7th above the root).

There are five basic types of seventh chords: major-major seventh (a.k.a. major seventh), minor-minor seventh (a.k.a. minor seventh), major-minor seventh (a.k.a. dominant seventh), diminished-minor seventh (a.k.a. half-diminished seventh), and diminished-diminished seventh (a.k.a. fully diminished seventh).

In a major-major seventh chord, the root, third, and fifth make up a major triad. The interval between the root and the seventh is a major 7th. The major triad and major 7th are the reason for the name major-major. This chord is more commonly referred to as a major seventh chord.

In a minor-minor seventh chord, the root, third, and fifth make up a minor triad. The interval between the root and the seventh is a minor 7th. The minor triad and minor 7th are the reason for the name minor-minor. This chord is more commonly referred to as a minor seventh chord.

In a major-minor seventh chord, the root, third, and fifth make up a major triad. The interval between the root and the seventh is a minor 7th. The major triad and minor 7th are the reason for the name major-minor. This chord is also referred to as a dominant seventh chord due to its association with scale-degree 5 in major and minor keys. (See Section 1 – Fundamentals: A.2.a. Scale Degrees.)

In a diminished-minor seventh chord, the root, third, and fifth make up a diminished triad. The interval between the root and the seventh is a minor seventh. The diminished triad and minor 7th are the reason for the name diminished-minor. This chord is more commonly referred to as a half-diminished seventh chord.

In a diminished-diminished seventh chord, the root, third, and fifth make up a diminished triad. The interval between the root and the seventh is a diminished 7th. The diminished triad and diminished 7th are the reason for the name diminished-diminished. This chord is more commonly referred to as a fully diminished seventh chord.