Mixolydian, and Why the Blues Breaks the Rules

The mode that explains the single most common anomaly in popular music.

Here is a problem the first theory path leaves you with. In the key of G, the chord built on the first degree should be G major, and the only dominant seventh in the key is D7 on the fifth degree. Yet in a blues in G, the very first chord is usually G7 — a dominant seventh on the I chord, which the theory says should not exist there.

It is not an error, and it is extremely common. G7 contains an F natural, and the key of G major has an F sharp. So a blues in G is not really using the G major scale at all. It is using G Mixolydian: a major scale with a flattened seventh.

G Mixolydian

G Mixolydian. Identical to G major apart from the seventh note, F natural instead of F sharp. That one flattened note is what lets the I chord be a dominant seventh.

Left-handed
0123456789101112131415EFGABCDEFGBCDEFGABCDGABCDEFGADEFGABCDEFABCDEFGABCEFGABCDEFG

Once you accept the flattened seventh as part of the key rather than an intruder, a second consequence follows immediately. If F natural belongs, then a chord can be built on it, and F major becomes available in the key of G. That is the flat-seven chord, written ♭VII, and it is everywhere in blues and rock.

This track is a blues in G, and both of those things are audibly in it — the I chord is a G7, and an F major turns up as the ♭VII.

G7

G7 as the I chord. In strict major-key theory this chord does not belong here; in Mixolydian it is the home chord.

321
G7 · position 1

F

F major, the ♭VII in the key of G. Built on the flattened seventh note of the scale.

1342
F · position 1
Rhythm Avenue l Groovy Blues Backing Track in G Major l BT 710
Backing Tracks Center#blues#Gmajor

A blues in G. The transcription for this track includes both G7 and F major — the two things this lesson is about, in a real recording.

One honest qualification, because this is where a lot of blues theory overreaches. Mixolydian explains the chords of a blues well. It does not fully explain the melodies, because blues players also use a flattened third over a major chord, and that note is not in Mixolydian either. The blues is genuinely a hybrid — major-ish chords with minor-ish melody notes rubbed against them — and no single scale accounts for all of it.

What Mixolydian does give you is a clean answer to the specific question of why the I chord is a seventh and why the ♭VII sounds right, which is the part that confuses people most.