Locrian Mode

Locrian is the seventh mode of the major scale and the most theoretically problematic one. It has a diminished 5th above the root — which means there's no stable perfect fifth to anchor the tonic chord. The tonic chord built from Locrian is a half-diminished chord (m7♭5), which by nature sounds unresolved. Locrian is hard to use melodically as a tonal centre because it refuses to feel like home. That instability is its only usable quality — and it's used exactly for that.

Construction

The seventh mode of the major scale — starts on the 7th degree.
B Locrian = C major scale starting on B:
B C D E F G A B

Formula:      1  ♭2  ♭3  4  ♭5  ♭6  ♭7
Step pattern: H  W   W   H  W   W   W

Compare to Phrygian: 1 ♭2 ♭3 4 5 ♭6 ♭7 (same, but with natural 5th)
Locrian takes Phrygian and also lowers the 5th — maximum flatness.

Locrian in Common Keys

Locrian root	Notes	Parent major key	The ♭5
B Locrian	B C D E F G A	C major	F
E Locrian	E F G A B♭ C D	F major	B♭
A Locrian	A B♭ C D E♭ F G	B♭ major	E♭
D Locrian	D E♭ F G A♭ B♭ C	E♭ major	A♭
F# Locrian	F# G A B C D E	G major	C
C# Locrian	C# D E F# G A B	D major	G

Sound and Character

Dark, unstable, and tense — uniquely so because the ♭5 removes the most stable interval (perfect 5th) from the tonic chord. The im7♭5 chord built on the Locrian root sounds half-diminished by nature: it already wants to resolve. Extended Locrian creates a sense of persistent unresolved tension — not building to a climax, just hovering in an uncomfortable space. This makes it useful for horror, avant-garde, and extreme metal contexts where sustained harmonic instability is the goal.

The Tonic Chord Problem

B Locrian tonic chord: B D F A = Bm7♭5 (half-diminished)
  B to D = minor 3rd ✓
  B to F = diminished 5th (tritone) ← the problem — not stable
  B to A = minor 7th ✓

The diminished 5th (B→F) makes the chord sound unresolved by definition.
There's no perfect 5th anchor — nowhere to land.

Compare:
  Bm7 (Dorian): B D F# A — the F# makes it stable
  Bm7♭5 (Locrian): B D F A — the F makes it inherently unstable

Practical Use: Over Half-Diminished Chords

The main practical application of Locrian is over m7♭5 (half-diminished) chords in jazz — specifically the ii° chord in a minor ii°-V7-i progression:

Minor ii-V-i in A minor:
  Bm7♭5 → E7 → Am

Over Bm7♭5: B Locrian
  Chord tones: B (root), D (♭3), F (♭5), A (♭7) — all in B Locrian ✓
  The scale matches the chord's character: both unstable, both half-diminished

Locrian ♮2 (a Better Option)

Many jazz players prefer Locrian ♮2 over m7♭5 chords. This is the 6th mode of the jazz melodic minor scale — same as Locrian but with a natural 2nd instead of a flatted 2nd:

B Locrian:   B C  D E F G A   (♭2 = C)
B Locrian ♮2: B C# D E F G A   (natural 2 = C#)

The natural 2nd removes the harshest tension and gives more melodic options.
B Locrian ♮2 = 6th mode of D melodic minor.

Both work over Bm7♭5. Locrian ♮2 is warmer; pure Locrian is more angular.

Comparison to Other Dark Modes

Mode	Formula	Darkest feature	Use case
Aeolian	1 2 ♭3 4 5 ♭6 ♭7	♭6 — melancholic	Standard minor — rock, classical
Phrygian	1 ♭2 ♭3 4 5 ♭6 ♭7	♭2 — exotic, tense	Flamenco, metal, exotic colour
Locrian	1 ♭2 ♭3 4 ♭5 ♭6 ♭7	♭5 — unstable tonic	Half-diminished chords, avant-garde

References

Levine — The Jazz Theory Book
Bergonzi — Inside Improvisation Series
jazzguitar.be — Locrian Mode ↗