Lydian

The note that defines it

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

The C♯ is a tritone above the tonic. In a major scale that interval demands resolution; over a static G chord it does not, and the difference between those two situations is what the mode is.

On the fretboard

Third fret with the root on the low E string, and open position works too, since only one note is altered from the natural set. Players who know D major shapes already have the fingering.

The trap is the C natural: it belongs to G major and not to this mode, and a phrase that includes it collapses the lydian sound instantly.

Where it is used

Film scoring and fusion over a Gmaj7 chord. In rock it appears when a progression alternates G and A major — the A supplies the C♯, and the mode follows from the harmony.

It is also common in progressive and instrumental guitar music, where a static major chord held for bars needs a scale that will not resolve it.

The mode also has a practical use in songwriting: holding a Gmaj7 chord and adding the C♯ to the melody produces the sound of expansion without any chord change at all, which is why film cues use it under wide shots.