As explained in Jazz Chords for Baritone Ukulele, there are four essential 7th chords. These are XMaj7, X7, Xm7 and Xm7(b5). The notes of each are:
Why are these considered the essential 7th chords? It’s all about chord scales.
Chord scales are composed of chords where the root of each chord is a note in the scale. So, in the key of C, the chord roots would be C, D, E, F, G, A, B. In other words, a C chord, a D chord, an E chord, etc. These are generically referred to with Roman numerals. (Sometimes lower case is used for minor chords but that shorthand really only makes sense when talking about simple major and minor chords, not 7th chords.)
In the key of C, for example, the I is a C chord, the IV is an F chord and the V a G chord.
In the key of D, the I is a D chord, the IV a G chord and the V an A chord.
The question is, which essential 7th chord to use for each of them? Which, for example, G chord to use for the V in the key of C?
GMaj7? G7? Gm7? or Gm7(b5)?
The answer is, a chord for which all of the notes in the chord are in the key.
Harmonies
Why? It’s all about the harmonies. A song in a particular key has a melody in which the notes are all, mostly, in the key. The melody moves horizontally through written music. As the melody moves, chords are chosen to work with it, providing harmony.
The harmony notes in music are written vertically. Just as the notes in the melody like to be within the key, so too do the notes of the harmony. And these are the notes which make up the accompanying chords.
In the following snippet of piano music you can see the melody line and the chord harmonies. The idea is that the notes of the melody are all in the key of C and the chord harmonies are all made of notes in the key of C as well (no sharps or flats).
Looking at the Notes
Consider the I chord in the key of C. It is, of course, a C chord. But which one has all of its notes in the key of C?
Only the CMaj7 works. Thus, the I chord of a chord scale is a major 7th.
What about the II chord in the key of C? It is a D chord. Which one has its notes in the key of C?
Only the Dm7 works. So the II chord of a chord scale is a minor 7th.
Skipping to the VII in the key of C, which B chord has its notes in the key of C?
So for the VII, only a minor 7th with a flatted 5 will do.
Continuing that analysis reveals the chords of a chord scale are thus:
These are the essential seventh chords you’ll often see accompanying a jazz song.
Looking at the Intervals
Another way to understand chord scales is by looking at the intervals that make up a chord. A chord is made up of stacked major (M3) and minor third (m3) intervals. Looking at these intervals lets you see why some chords are in the scale, and others are not.
Here are the intervals of a Maj 7th chord laid over a scale, with the root in the first note. Note how each note in the chord lines up with a note in the scale.
We can slide that pattern up so the root is on the 4 note of the scale making the IV chord.
The notes still line up.
We can then try the pattern over the 5 note of the scale, making a V chord.
Note that the notes do NOT line up, which means a Maj 7th chord is not in the scale for the V chord. If we try to use the Maj7th shape for the V chord, it will sound dissonant because the 7 of the chord lines up between the 4 and 5 of the scale, not on a note.
But, if it were a b7, as in a 7th chord…
That would work! So a 7th chord is in the scale with the 5 as the root. In other words, V 7 is in a chord scale. Curiously, in a genre so reliant on 7th chords, the actual 7th chord only works as a V.
The chords that start with a major 3rd (M3) won’t work for any other chord because the rest all need to start with a minor 3rd (m3) interval. The minor 7th shape works well though. Here it is on the II.
And on the III.
And on the VI.
And on the VII.
Oops, the m7 doesn’t work for VII! The 5 of the chord fails to line up with a note in the scale.
But if we just flat that 5… And this is why the m7(b5) is so important. It’s all about chord scales and making sure all the notes in the chords are harmonious with the key of the song.
That’s chord scales!
