The Locrian scale is a minor scale with a flat 2nd and a flat 5th: the darkest, most unstable of the modes. It's rarely a key center; its real job is soloing over m7b5 chords, plus heavy metal riffs.
Formula
| 1 | b2 | b3 | 4 | b5 | b6 | b7 |
|---|---|---|---|---|---|---|
| A | Bb | C | D | Eb | F | G |
Steps: H W W H W W W. The reference key is A Locrian, and it's what the diagrams below use. It's the seventh mode of Bb major, so it shares its notes with Bb major and G natural minor. Compared with A Phrygian (A Bb C D E F G), only one note changes: E drops to Eb.
Sound and character
The b5 is the problem and the point. Without a perfect 5th, the root never sounds settled, so the scale always seems to be heading somewhere else. Over its own chord it sounds tense, dark and suspended rather than sad.
Where it's used
Locrian is rare as a home key: you'll almost never play a whole song "in A Locrian". You'll use it over m7b5 chords, above all the ii chord in a minor key: Am7b5 in a ii-V-i to Gm (Am7b5, D7, Gm). Jazz, bossa nova and minor ballads are full of that move. In metal, riffs grind the b2 and b5 against a low root for a sound that refuses to resolve. The chords built from the scale are Adim, Bb, Cm, Dm, Eb, F and Gm; as 7ths, Am7b5, Bbmaj7, Cm7, Dm7, Ebmaj7, F7 and Gm7.
How to move it
The root is the amber dot. Slide the whole pattern so the amber dots land on the new key's root: move everything up 2 frets for B Locrian (Bm7b5, the vii chord of C major), or up 7 frets for E Locrian, the ii chord of D minor.
Related scales: Locrian is Phrygian with a b5. Raise its 2nd and you get Locrian #2, a mode of melodic minor that jazz players also use over m7b5.
The second exercise plays Am7b5, its arpeggio, then a scale phrase that ends on the b5.