I’ve had students who’ve emailed me in the past asking questions like “is it a min7b5 or a half-dim7 chord?.”
In this lesson, I’ll answer that question satisfactorily, give you more insights and also provide you with the theoretical basis of my answer.
There are two known names for the seventh chord of the seventh tone of the scale (aka – “the subtonic chord”):
The half-diminished seventh chord
The minor seventh (flat five) chord
In the key of C major:
…the subtonic chord consists of B, D, F and A:
Attention: At this point and for the rest of the beginning part of this lesson, we’ll rather describe it as the subtonic chord and this is because the technical name of seventh tone of the scale is the subtonic.
The Structure Of The Subtonic Chord
You’ll no longer doubt the appropriate name for the subtonic chord in the major key if you take a look at its structure. You’ll just know what the appropriate name should be.
A Breakdown Of The Subtonic Chord
The subtonic chord can be broken down into two chords: a triad and a seventh chord. The subtonic chord in the key of C major:
…which is “B-D-F-A”:
…can be broken down into the following chords:
B-D-F:
B-D-F-A:
The former is a triad while the latter is a seventh chord.
Min7b5 Vs Half-dim7
The breakdown of the subtonic chord revealed a triad (B-D-F):
…and the triad has vital information that can help us determine if the subtonic chord is a min7b5 chord or a half-dim7 chord.
The 7-chord (aka – “subtonic chord”) at the triadic level is a diminished chord and B-D-F (which is a diminished triad):
…is no exception.
Now, if the subtonic chord at the triadic level (B-D-F):
…is a diminished triad, what do you think it would be at the seventh chord level (B-D-F-A):
The answer is clear:
It would be a half-diminished seventh chord
The structural breakdown of the subtonic chord shows that it’s a diminished chord and this is clear at the triadic level. So, the subtonic chord is clearly NOT a minor chord (min7b5 chord).
The Function Of The Subtonic Chord
Beyond the structure of the subtonic chord, one other pointer to its appropriate name is its function. A proper understanding of the function of the subtonic chord is the key to knowing if the subtonic chord is a min7b5 chord or a half-dim7 chord.
“What Is The Function Of The Subtonic Chord…”
The subtonic chord in the major key is also known as the leading note chord and this is because of its dissonance.
When played, the leading note chord resolves upwards by a half-step to the 1-chord in the key as seen in this example between the leading note chord and the 1-chord in the key of C major:
The leading note chord:
The 1-chord:
So, the subtonic chord by virtue of its resolution to the 1-chord in the key functions as a leading note chord.
Min7b5 Vs Half-dim7
If the subtonic chord doesn’t have a leading note role in the major key, it would have been safe to assume that subtonic chord is a minor seventh chord with a flattened fifth tone.
It’s just like saying that the 7-chord (subtonic chord):
…is a B minor seventh chord:
…with a flattened fifth (from F# to F):
F#:
…to F:
…to produce B-D-F-A (the minor seventh chord with a flattened fifth).
But from the leading note function of the subtonic chord in the major key, it’s clear that the subtonic chord has nothing to with a minor chord (min7b5) and everything to do with a diminished chord (half-dim7).
In tonal music, only diminished chords can function as leading note chords (neither major nor minor chords can) and that’s why the use of the name min7b5 to describe the subtonic chord a misnomer.
Final Words
Half-diminished seventh chords can be associated with minor seventh chords and it is this association that produced the name, min7b5. Lowering the fifth tone of any given minor seventh chord by a half-step produces the half-diminished seventh chord.
So, it’s a half-diminished seventh chord and the name “min7b5” can only serve the ordinary purpose of teaching students how to play the chord but isn’t the appropriate name of the chord.
See you in the next lesson.
Chuku Onyemachi
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