Ultra Locrian (Altered Diminished bb7) is the most extreme mode of harmonic minor. With a bb7 (enharmonic to the 6th), it is fully diminished in character. Rarely used as a standalone scale but important for understanding the complete harmonic minor system.
Scale Formula
1 b2 b3 b4 b5 b6 bb7 - everything flattened, including double-flat 7. Semitone pattern: H W H W W H WH
Fretboard (C Ultra Locrian)
Toggle to interval view to see the extreme alteration. The bb7 (enharmonic A) sits where you would expect the 6th, making this mode symmetrically diminished in character.
Common Chords
Diminished 7th (1 b3 b5 bb7), Diminished triad (1 b3 b5).
Diatonic Harmony
Improvisation Context
Play over diminished 7th chords. This is a theoretical mode - in practice, diminished 7th chords are more commonly approached with symmetric diminished scales (whole-half or half-whole).
Practice
Play C Ultra Locrian over a Cdim7 chord and compare it with the half-whole diminished scale (which has 8 notes instead of 7). The symmetric diminished scale is usually more practical, but understanding Ultra Locrian completes your map of harmonic minor. This mode is primarily for theoretical fluency - knowing where it lives helps you navigate the full harmonic minor system.