The Locrian scale is one of the seven musical modes and is the most unstable and dissonant of them all. It's characterized by its diminished fifth (or tritone) interval, which gives it a tense and unresolved sound. Because of this, it's less commonly used as the primary scale in compositions but is still an important tool for creating certain moods or atmospheres.

Construction:

The Locrian scale is built using the following interval pattern:

• Half step

• Whole step

• Whole step

• Half step

• Whole step

• Whole step

• Whole step

In B Locrian, it would be:

• B (root)

• C (half step)

• D (whole step)

• E (whole step)

• F (half step)

• G (whole step)

• A (whole step)

• B (octave)

Features:

• Minor tonality: The Locrian mode has a minor tonality, characterized by a minor third (between the root and the third degree).

• Diminished fifth: The defining feature of the Locrian mode is its diminished fifth (or tritone), which is a dissonant interval that creates tension and instability.

• Flat second, third, fifth, sixth, and seventh degrees: Compared to the major scale, the Locrian mode has a flat second, third, fifth, sixth, and seventh, contributing to its dark and dissonant sound.

Chord Construction:

• Tonic chord (i°): B diminished (B, D, F)

• Supertonic chord (ii): C minor (C, E♭, G)

• Mediant chord (III): D♭ major (D♭, F, A♭)

• Subdominant chord (iv): E♭ minor (E♭, G♭, B♭)

• Dominant chord (v): F minor (F, A♭, C)

• Submediant chord (VI): G♭ major (G♭, B♭, D♭)

• Subtonic chord (VII): A♭ major (A♭, C, E♭)

Usage:

• Classical Music: While rarely used as a standalone scale, the Locrian mode is sometimes employed in classical music to create tension, unease, or a sense of unresolved conflict.

• Jazz: In jazz, the Locrian mode is often used over half-diminished (m7♭5) chords, providing a dissonant, tense atmosphere that leads to resolution.

• Metal and Experimental Music: The dark, unstable quality of the Locrian mode makes it attractive for metal and experimental music, where it can be used to create an ominous or chaotic sound.

• Film Scores: Composers might use the Locrian mode in film scores to underscore scenes of danger, suspense, or other unsettling emotions.

The Locrian scale's inherent instability and dissonance make it a powerful tool for creating tension and unease in music, though its unique sound means it's often used sparingly and with specific intent.