Chord Theory 2: Seventh Chords

In the last chord families post we looked at every possible three note combination of stacked major and minor thirds. Today we’re going to look at each of the four note combinations.

Building Four Note Chords

Four note chords are built by stacking an extra third (major or minor) on top of the triads already covered in part 1. The table below shows every possible combination of four note chords built in thirds. (Although you will notice that that there is no augmented triad with a major third on top. This is because augmented chords are symmetrical chords, which is something that will be discussed in the next post.)

Chord Type	Basic Triad	Top Third	Chord Symbol(s)
Major 7th Chord	Major Triad	Major Third	M7, maj7, Δ
Dominant 7th Chord	Major Triad	Minor Third	7
Minor-major 7th Chord	Minor Triad	Major Third	min(maj)7, min/maj7, mi/MA7, -Δ
Minor 7th Chord	Minor Triad	Minor Third	m7, -7
Half-diminished Chord (aka Minor 7 b5 Chord)	Diminished Triad	Major Third	ø, m7b5,-7b5
Diminished 7th Chord	Diminished Triad	Minor Third	o7
Augmented Major 7th Chord	Augmented Triad	Minor Third	maj7(♯5), maj+7, and Δ+7.

Major Seventh Chords

From chord theory part one we know that a major triad is constructed of a major third plus a minor third. Adding another major third on top will give us a major seventh chord. Therefore the interval structure of a major seventh chord is major third, minor third, major third.

The figure below shows the thirds structure applied to the note C, resulting in a C major seventh chord.

The following figure relates each note of the C major seventh chord to the root to find the chord formula. C is the Root; E is a major third above C; G is a perfect fifth above C; and B is a major seventh above C. This gives us the major seventh chord formula: 1, 3, 5, 7.

Dominant Seventh Chords

The dominant seventh chord is a major triad with a minor third on top. So the thirds structure of a dominant seventh chord is major third, minor third, minor third.

The figure below relates each note of a dominant seventh chord to the root note, C, giving us the dominant seventh chord formula: 1, 3, 5, b7.

Minor Seventh Chords

Taking a minor triad and adding a minor third on top creates the minor seventh chord. A minor triad is a minor third plus a major third, therefore the structure of a minor seventh chord is minor third, major third, minor third.

Relating every note to the root, we arrive at the chord formula: 1, b3, 5, b7.

Minor-major Seventh Chords

Minor-major seventh chords are a minor triad with a major third on top. The interval structure of a minor-major seventh chord will therefore be minor third, major third, major third.

From this C minor-major seventh chord we can derive the minor-major seventh chord formula: 1, b3, 5, 7.

A min/maj seventh chord gets its name from the fact that it is a minor triad, but unlike the minor seventh chord it has a major seventh on top.

Half Diminished (Minor Seventh Flat Five) Chords

Stacking a major third on top of a diminished triad creates the half-diminished chord – also known as a “minor seven flat five” chord. A diminished triad is constructed of two stacked minor thirds, so the structure of a half-diminished chord is minor third, minor third, major third.

From this C Half-diminished seventh chord we arrive at the chord formula: 1, b3, b5, b7.

The name minor seventh flat five comes from the fact that the chord formula is the same as the chord formula for the regular minor seventh chord but with the fifth flattened (the minor seventh formula is 1, b3, 5, b7, whereas the minor seventh flat five is 1, b3, b5, b7)

The name half-diminished comes from the similarity of the chord formula with the diminished seventh chord formula (shown below). The diminished seventh chord has two diminished intervals in its chord formula, however the half-diminished chord only has one diminished interval (the b5) – making it only ‘half’ diminished compared to the regular diminished seventh chord.

Diminished Seventh Chords

A diminished triad plus a minor third creates the diminished seventh chord. Every interval in a diminished seventh chord is a minor third, so the structure is minor third, minor third, minor third.

When building a C diminished chord it is very important that the top note is written as a B double flat. Chords are built in thirds, and a Bbb is accordingly a minor third above Gb (three letter names). Spelling the Bbb as its enharmonic equivalent, the note A, will cause problems later when the theory gets more involved (Gb to A is not a minor third, it is an augmented second).

From this C diminished seventh chord we arrive at the chord formula: 1, b3, b5, bb7.

Note that the seventh degree is a double flattened seventh (bb7). Double flattened sevenths are also known as diminished sevenths, and is where this chord gets its name from.

Augmented Major Seventh

Starting with an augmented chord and adding a minor third on top results in the augmented major seventh chord. The thirds structure of augmented triads is two stacked major thirds, so adding a minor third on top to create an augmented major seventh gives the structure major third, major third, minor third.

The chord formula is therefore: 1, 3, #5, 7.

Looking at the chord formula we can see that the augmented major seventh is a regular augmented chord but with a major seventh on top. This is what gives the augmented major seventh its name.

Summary

Here’s a table showing everything covered in this post.

Chord Type	Interval Structure	Chord Formula	Chord Symbol(s)
Major 7th	Major Third, Minor Third, Major Third	1 3 5 7	M7, maj7, Δ
Dominant 7th	Major Third, Major Third, Minor Third	1 3 5 b7	7
Min/Maj 7th	Minor Third, Major Third, Major Third	1 b3 5 7	min(maj)7, min/maj7, mi/MA7, -Δ
Minor 7th	Minor Third, Major Third, Minor Third	1 b3 5 b7	m7, -7
Half-diminished	Minor Third, Minor Third, Major Third	1 b3 b5 b7	ø, m7b5,-7b5
Diminished 7th	Minor Third, Minor Third, Minor Third	1 b3 b5 bb7	o7
Augmented Major 7th	Major Third, Major Third, Minor Third	1 3 #5 7	maj7(♯5), maj+7, and Δ+7.