Locrian mode

Locrian mode, in Western music, the melodic mode with a pitch series corresponding to that produced by the white keys of the piano within a B–B octave.

The Locrian mode and its plagal (lower-register) counterpart, the Hypolocrian mode, existed in principal long before they were mentioned by the Swiss humanist Henricus Glareanus in his landmark music treatise Dodecachordon (1547). In that work Glareanus expanded the standing system of church modes to accommodate the increasingly common major and minor modes as well as the growing importance of harmony as a determinant of melodic motion. However, the Locrian and Hypolocrian modes were notably excluded from the corpus of available modes because their finalis (the tone on which a piece in a given mode ends) on B, when paired with their secondary centre on F, created a tritone. Also known as the diabolus in musica (“devil in music”), the tritone was generally a forbidden sonority until the 18th century.