Locrian mode
The seventh mode of the major scale, built on the seventh degree, characterised by a diminished fifth above the tonic
In Depth
Locrian is the darkest and most unstable of the seven diatonic modes. Its defining feature is the diminished fifth (tritone) between the first and fifth degrees, which prevents any stable tonic triad from forming. This instability makes Locrian extremely rare in traditional Western music, though it appears in some metal, jazz, and experimental contexts. Its interval pattern is H-W-W-H-W-W-W. Some theorists argue that Locrian is not a true mode because it cannot support a functional tonic.
Locrian is the only diatonic mode that was not used in medieval plainchant, and some theorists excluded it from the modal system entirely.