E Locrian — Puerto Rican Cuatro (Antiguo A-D-G-C-F)

E Locrian is 7 notes to the octave (modes family), drawn here across the Puerto Rican Cuatro (Antiguo A-D-G-C-F)'s A-D-G-C-F strings with every E marked, so home stays visible from anywhere in the pattern.

The most dissonant, unstable mode — the tonic chord is diminished (b5), making a true Locrian tonal center rare. Used for color and passing harmony.

Heard in: Metal riffs, jazz (half-diminished ø7 chords in ii°7 – V – i), horror and tension film scores.

Five courses spanning a minor thirteenth give a scale room to breathe: three or four notes to a course in first position, and a top string clear enough that a melody can live there while the rest of the grip stays down. These maps light every root so a run knows where home is before it leaves.

Work it in one position before you work it up the neck. The doubled courses forgive nothing: a fret held half a millimetre short buzzes on one string of the pair and rings on the other, and that difference is audible long before the run is fast.