Mixolydian

The notes

C D E F G A B♭, then C. It is the fifth mode of F major and shares that key’s single flat. Playing F major from C produces it directly.

The B♭ is one semitone from the B that C major uses, and that semitone is the entire difference between a scale that leads home and one that already is home.

On the fretboard

The eighth-fret position with the root on the low E string, or the third fret with the root on the A. Neither uses open strings, so both move without alteration.

For soloing, the practical route is the C major pentatonic with the B♭ added — the added note is what converts the sound, and leaving it out returns the scale to plain major.

Where it is used

Over C7 in a blues in C or F, and over funk vamps that never change chord. In both cases the flattened seventh is what allows the harmony to stand still.

In jazz it is the scale over the V of F major. Because C7 is the most common dominant in horn keys, this mode appears in the repertoire far more often than its guitar-friendliness would suggest.

It also explains why so many rock songs in C use a B♭ chord: that chord belongs to the mode and not to the key, and its presence is the clearest sign the music is mixolydian rather than major.