Locrian bb7

The diminished seventh chord

Stacking thirds from the root gives C E♭ G♭ B𝄫 — a diminished seventh, four notes each a minor third apart. No other mode of harmonic major produces this chord, and describing it is the reason the scale is catalogued at all.

The doubly flattened seventh is written that way and not as a sixth because of the chord it belongs to. On an instrument it is the same pitch as A natural; on paper it has to be a seventh, or the chord above it stops reading as a diminished seventh and starts reading as a minor sixth.

Not the altered scale with a bb7

A second scale carries a double-flat seventh as well: the altered diminished, seventh mode of harmonic minor, sometimes printed as super Locrian bb7. That one is 1 b2 b3 b4 b5 b6 bb7, and the difference from this scale is the fourth — flattened there, natural here.

One note, two families, two different parent scales. If a chart calls for a bb7 scale over a diminished chord, check which fourth it wants before choosing; the natural fourth of Locrian bb7 is much less tense and will not supply the altered sound the other one gives.

Where the augmented second falls

In this mode the family's augmented second sits at the very top, between the flat sixth and the doubly flattened seventh: A♭ up to B𝄫 is three semitones. Descending through the top of the scale therefore drops a wide interval before reaching the root, which is the gesture that identifies the mode by ear.

Everything below that is a chain of semitones and whole tones, and the lower five notes on their own could belong to several scales. The top interval is the only unambiguous landmark, which is worth knowing when learning the fingering.

How it is used

Fully diminished seventh chords in a progression are the natural target, particularly passing diminished chords between two diatonic chords. The scale supplies every chord tone plus a flat ninth, a natural eleventh and a flat thirteenth.

It is a one-chord scale and a short-lived one: diminished chords rarely last more than a beat or two. The practical use is not to run the scale but to know which two non-chord tones are available above the chord, and to place them where the line turns.

Frequently asked questions