Diatonic Chords (Four-Part Harmony)

Let's study four-part chords.
This is because in practice, four-part chords are used more often.
Just as with three-part chords, we stack notes in thirds starting from the root.

Musical Example 1

How to read

M7... Major Seven (alternative notations include maj7, Δ7)

m7... Minor Seven (alternative notation includes -7)

7... Seven

m7(♭5)...
Minor Seven Flat Five
or
Half-Diminished (alternative notation includes φ)

Things that tend to be ambiguous

Many people have a vague understanding of the differences between M7, m7, and 7, so let's organize them.
M7 refers to a note that is a major seventh interval from the root note. 7 refers to a note that is a minor seventh interval from the root note.

How can the notation be broken down?

CM7... C and M7 (adding a major seventh note to a C major chord)
Dm7... Dm and 7 (adding a seventh note to a D minor chord)
G7... G and 7 (adding a seventh note to a G major chord)

Since the seventh note is quite far from the root, it is difficult to distinguish whether it is a major seventh (M7) or a minor seventh (7).
However, you can easily distinguish them by thinking as follows.

Musical Example 2

If you raise the root note by one octave, it becomes a second interval relationship, adjacent to the seventh note from the root.
If the interval at that time is a semitone (minor second), it is a major seventh (major seventh); if it is a whole tone (major second), it is a minor seventh (seventh).
The major seventh is further from the root than the minor seventh.
In Musical Example 2, you should be able to see the difference between CM7 and C7.

Musical Example 3

Musical Example 3 is a variation of four-part chords with C as the root note.
The one on the far right has the long name C minor major seven, but if you understand the structure of the chord, it is a name that makes perfect sense.

Difference between m7(♭5) and diminished

Also pay attention to the m7(♭5) that is formed on the seventh degree of the diatonic scale.
Here, for the sake of clarity, we will use C as the root.
There is a chord with a similar sound called a diminished chord.
A diminished chord is a chord that

consistently stacks minor thirds.Therefore, at the stage of stacking up to three notes, there is no distinction between m7(♭5) and diminished, and both are called diminished.
However, a difference appears at the fourth note.While m7(♭5) adds the B-flat note, which is the seventh, the diminished chord is thought of as adding a minor third above the third note, G-flat, so the fourth note becomes A. (See Musical Example 4)
Musical Example 4