Mixolydian

The notes

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

That note is also what makes the scale usable over a dominant chord: the flattened seventh belongs to F7 by definition, and the scale simply spells out what the chord already contains.

On the fretboard

First fret with the root on the low E string, or eighth with the root on the A. As with everything in F, the higher position is the practical one.

Because nothing is open, phrasing depends entirely on bends and slides. That is a difference from E or A mixolydian, where open strings do part of the work.

Where it is used

Blues and jazz in F, which is a horn key and therefore common in ensemble playing. It is also the scale over the V of B♭ major, one of the most-used dominants in the standard repertoire.

For guitarists the mode arrives with the chart rather than by choice, and knowing it at the eighth fret is usually enough to play through a session without difficulty.

The mode also shows why horn keys feel foreign on guitar: the notes are ordinary, but none of them rings open, and the whole scale has to be produced by the left hand rather than by the instrument.