The Locrian mode is the darkest sound you can pull from the major scale. The b5 gives it a diminished quality - restless and unresolved, with no stable home chord to land on. That instability is exactly what makes it useful: Locrian is the go-to scale for half-diminished chord contexts.
Scale Formula
1 b2 b3 4 b5 b6 b7 - everything flatted except 4. Semitone pattern: H W W H W W W
Fretboard (C Locrian)
Use the interval toggle above the fretboard to see how many flatted degrees Locrian packs in. The b5 stands out as the defining one - it turns what would be a minor triad into a diminished triad.
Common Chords
Diminished triad (1 b3 b5), Half-diminished 7th / Minor 7b5 (1 b3 b5 b7).
Diatonic Harmony
Improvisation Context
Play over half-diminished (m7b5) chords, typically functioning as a vii chord in major keys. The b5 and b2 define its character.
Characteristic Note
The b5 is what makes Locrian uniquely unstable. Every other mode has at least a minor or major triad built on the root, but Locrian's root triad is diminished - there is no restful "home" chord. Where this actually becomes useful is over the ii chord in minor key ii-V-i progressions. For example, Dm7b5 in C minor is a textbook spot for Locrian.
Practice
Play C Locrian over a Cm7b5 chord and listen for the tension the b5 (Gb) creates against the root. Once you have that sound in your ear, put it in context: play through a Cm7b5 – F7 – Bbm progression (ii-V-i in Bb minor) and use Locrian over the Cm7b5. For a revealing comparison, try Locrian #2 (from melodic minor) over the same chord - the natural 2 smooths out the roughest edge, and in jazz contexts, many players prefer it.