Lydian

The note that defines it

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

On guitar the D♯ sits at the first fret of the D string or the sixth of the A — awkward reaches in a key whose other notes fall easily, which is part of why the mode takes practice.

On the fretboard

Fifth fret with the root on the low E string. The open position works partially, since A, B, E and the open high E belong to the scale, but the D♯ always has to be fretted.

The danger is the D natural, which the hand plays automatically because A major is so familiar. One such note and the lydian character disappears entirely.

Where it is used

Film scoring and instrumental rock over an Amaj7 chord. It also appears when a progression alternates A and B major — the B supplies the D♯, and the mode follows from the harmony.

In jazz it is the scale over the IV chord in E major, which makes it more common in that key than its own signature would suggest.

For a guitarist the practical route is through E major shapes rather than A major ones. Thinking of the scale as E major starting from A removes the temptation to play the D natural that the familiar key supplies.