Lydian

The note that defines it

F G A B C D E, then F. It is the fourth mode of C major and uses only natural notes, which makes F lydian the easiest lydian to play and the clearest to hear.

Compare with F major, which has B♭. The raised fourth removes the strongest downward pull in the major scale and leaves the whole thing floating.

On the fretboard

Open position works because every note is natural. The first-fret position with the root on the low E gives the movable shape, and the eighth fret repeats it more comfortably.

The B natural sits where a player expects B♭, which is why the mode takes practice: the hand has learned F major and has to unlearn one note.

Where it is used

Film and television scoring above all — the raised fourth over a major chord is the standard sound of wonder in that idiom, to the point of cliché.

In jazz it is the scale over a IV chord, and in rock it appears whenever a progression alternates F and G major without a C in sight.

It is also the easiest mode to demonstrate: play a sustained F major chord and run the white notes above it. The B natural sounds bright rather than wrong, and that is the whole lesson.