Mixolydian

The note that defines it

A B C♯ D E F♯ G, then A. Compare with A major, which has G♯. The natural G is the whole difference, and it turns the tonic chord from A into A7 without anything sounding unresolved.

The scale is the fifth mode of D major, so its notes are identical to that key. Playing D major from A is the quickest way to find it.

On the fretboard

Open position works well: A, B, D, E and G are all available open or at the second fret. The fifth-fret position with the root on the low E string is the movable form.

Because the tonic chord is a dominant seventh, this is the scale that fits a blues or rock riff sitting on A7 without wanting to move — which describes a large share of the repertoire.

Where it is used

Rock, blues and folk over a static dominant chord. Anything built on A7 that never resolves to D is mixolydian, whether or not the player knows the word.

In jazz it is the default scale over a V chord, which makes it the second mode most improvisers learn after dorian.

The mode also explains a common confusion. A song in A that never resolves to D is not in D major even though it shares the notes — the tonic is where the music rests, and mixolydian is the name for resting on the fifth degree.