Lydian

The note that defines it

D E F♯ G♯ A B C♯, then D. It is the fourth mode of A major and shares its three sharps. Compare with D major, which has G natural.

The G♯ is a tritone above the tonic, and against a static D chord it sounds bright rather than unstable — the same effect every lydian mode produces, heard here in the guitar’s most resonant major key.

On the fretboard

Tenth fret with the root on the low E string, or fifth fret with the root on the A. Open position works partially: D, E, A and B are open or nearly so, but the G♯ has to be fretted every time.

The trap is the G natural, which belongs to D major and not to this mode. A single such note collapses the lydian sound, and it is the note the hand plays automatically.

Where it is used

Film scoring over a Dmaj7 chord, and progressive rock where a major key needs to sound expansive rather than resolved.

In songs it usually appears through the chords rather than the melody: a progression that alternates D and E major contains the G♯, and the mode follows whether or not anyone names it.

It is also one of the few modes where the guitar helps: the open D and A strings belong to the scale, so a drone underneath is available without fretting, and the raised fourth sounds against it clearly.