Locrian bb7 is the darkest mode of harmonic major. With a double-flat 7th (enharmonic to the 6th), it is fully diminished in character. Like Ultra Locrian from harmonic minor, it is primarily theoretical but completes the system.
Scale Formula
1 b2 b3 4 b5 b6 bb7 - Locrian with double-flat 7. Semitone pattern: H W W H W H WH
Fretboard (C Locrian bb7)
Toggle to interval view to see the bb7 and all the flatted degrees. This mode is almost fully diminished - only the natural 4 remains unaltered.
Common Chords
Diminished 7th (1 b3 b5 bb7), Diminished triad (1 b3 b5).
Diatonic Harmony
Improvisation Context
Play over diminished 7th chords. As with Ultra Locrian, in practice diminished chords are usually approached with symmetric diminished scales. This mode is mainly important for understanding the harmonic major system completely.
Characteristic Note
The bb7 is the defining alteration. Regular Locrian already has b2, b3, b5, b6, and b7 - lowering the 7th further to bb7 creates a diminished 7th chord rather than a half-diminished one. The bb7 is enharmonically the same as the natural 6th, but functions as a further-flattened 7th in the context of the scale. The augmented 2nd between bb7 and root completes the harmonic major system's signature interval.
Practice
Play C Locrian bb7 ascending and descending. Compare it with regular C Locrian -- the only difference is the 7th degree (Bb vs Bbb/A). Over a Cdim7 drone, play through the scale slowly and notice how the bb7 pulls strongly toward the b6 below. Then compare with the whole-half diminished scale to hear how a symmetric approach differs from this modal one.