The problem it solves
Every other mode has a perfect fifth that supports the tonic. Locrian does not: its b5 destroys that support and leaves the tonic without a stable rest. Understanding Locrian means understanding why a mode can sound unfinished and why it rarely acts as a tonal centre.
Detailed theory
Key idea
Locrian has a diminished 5th (b5), so the tonic triad is diminished and offers no rest; the step pattern is S-T-T-S-T-T-T.
It is the most unstable and dissonant mode: it rarely acts as a real tonal centre and is used mostly over the ii chord (m7b5) of a minor key.
Understand it
Locrian is the mode you get by playing a major scale starting and ending on its seventh degree. That is why B Locrian uses exactly the notes of C major, but with B as the centre: B-C-D-E-F-G-A-B, all white keys. It has a minor third (D), so it is a minor mode, but with two lowered notes that make it unique.
The note that changes everything is the fifth. Counting from B, the fifth is F: a diminished fifth, not a perfect one. Every other mode has a perfect fifth that gives stability and support to the tonic; Locrian, instead, puts a diminished fifth, and that is why the triad built on the tonic (B-D-F) is diminished. A diminished triad does not rest: it wants to move, and that leaves the mode without a clear rest.
Locrian also has a minor second (C), like Phrygian, which adds the friction of the half step right above the tonic. The combination of b2 and b5 is what makes Locrian so dark and unstable: neither does the tonic find support (because of the b5) nor does the base sound calm (because of the b2).
The tone-and-semitone pattern of Locrian is S-T-T-S-T-T-T: it starts with a semitone (B-C), like Phrygian, and carries the second semitone between the 4th and 5th degrees (E-F), which is what lowers the fifth. Those two narrow steps are the signature of the mode.
For all this, Locrian is more a theoretical and colour tool than a mode to build a stable melody on. In practice it appears mostly over the m7b5 chord (the ii of a minor key), as improvisation material or to give a moment of very dark tension. Mastering Locrian means understanding why the b5 breaks the stability and knowing how to use it as an occasional colour, not as home.
Interval distance
B→F: the Locrian diminished fifth (6 semitones), not the perfect fifth (7). This lowered fifth is what makes the tonic triad diminished and leaves the mode without stable support.
How to recognise it
How it's written
Look at the tonic and its fifth: if the fifth is diminished (not perfect) and there is also a minor second over a minor environment, it is Locrian. In B Locrian, on the keyboard, you see it at once: only white keys with B as the centre, and the F acting as the diminished fifth.
How it feels
Play the tonic and its fifth together (B and F): it does not sound stable like a perfect fifth, but tense and suspended. That lack of support, together with the half step of the minor second, is the unstable audible signature of Locrian.
Common mistake
Treating Locrian as a stable tonal centre: its diminished fifth gives no rest, which is why it rarely acts as a real tonic.
Confusing Locrian with Phrygian: both have the minor second (b2), but Locrian also has the diminished fifth (b5); that fifth is the key difference.
Try it
On the keyboard, play B-C-D-E-F-G-A-B (white keys only) and notice that the F, the fifth, is diminished relative to B.
Play B and F together and compare with B and F#: the perfect fifth (F#) rests; the diminished one (F) stays tense and unstable, and that is the heart of Locrian.
On the instrument
Staff & keyboard
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The whole of B Locrian (white keys only, with B as the centre). The F, marked as the diminished fifth (b5), is what removes the support from the tonic; the C is the minor second (b2). B is the tonic, but with no stable rest.
Generate a phrase in this mode
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Generate fresh phrases in this mode, in any key, to explore its sound.
Where it's used
- Colour over half-diminished chords
- Improvising or writing over the m7b5 chord (the ii of a minor key), where Locrian fits with its diminished fifth.
- Occasional dark tension
- Adding a moment of great instability and suspension, knowing that Locrian does not rest and pushes towards a resolution.
- A theoretical comparison tool
- Understanding why the perfect fifth supports the tonic, making visible by contrast what happens when it is lowered.
Examples
Chord progression
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Locrian lives over the diminished triad of its tonic: B diminished (B-D-F) wanting to resolve to C major. The diminished fifth (the F) removes the rest and pushes the motion; that is why Locrian sounds unfinished.
Exercises
Locrian melodic dictation
Transcribe short phrases in the Locrian mode to internalise its diminished fifth and instability.
Complete 6 attempts · 70% accuracy to pass
Phrases in Locrian
Read and play phrases in the Locrian mode to fix its unstable, dissonant sound.
Complete 6 attempts · 70% accuracy to pass
Mini test
Check that you've got it.
0/6 answeredQuestion 1/6
Which note makes the Locrian mode so unstable?