Phrygian

The note that defines it

G A♭ B♭ C D E♭ F, then G. It is the third mode of E♭ major and shares its four flats. Against G natural minor, which has A natural, the single lowered second is the whole difference.

The A♭ presses on the tonic from above. In a natural minor the second degree supports the root; here it crowds it, and that crowding is what the mode is for.

On the fretboard

Third fret with the root on the low E string, or tenth with the root on the A. Nothing is open, so the characteristic semitone has to be fretted with two fingers rather than played against a ringing string.

That is a real difference from E phrygian, where the open low E and the first fret above it give the same interval for free. The G version demands more and sounds tighter for it.

Where it is used

Metal in tunings that drop the sixth string to G, where the mode arrives with the tuning. It also appears in film scoring built on flat keys.

In jazz it is the scale over a iii chord in E♭ major — a role it shares with every phrygian mode and fills about as rarely.

The comparison with E phrygian is worth making deliberately: identical intervals, entirely different effort, and a noticeably tighter sound because nothing rings open. The mode is the same; the instrument is not.