The Mixolydian Mode: How to Use It Over Dominant Chords
The Mixolydian mode (see [[guitar-modes-without-seven-boxes:the article on modes without seven boxes]]) is, in practice, the major scale with one change: the seventh degree flattened by a semitone. That single note lines up directly with the note that makes a dominant seventh chord dominant.
One note versus a plain major scale
Mixolydian matches the major scale (see the article on connecting chords and scales) in everything except the seventh degree - natural in major, flattened by a semitone in Mixolydian. Same logic that separates Dorian from the natural minor (see the article on the Dorian mode), just for the major family.
Why that exact note matters over G7
A dominant seventh chord like G7 (see the article on intervals) physically contains exactly that flattened seventh degree - a plain major scale doesn't contain it at all. Playing plain G major over G7, a guitarist either avoids the seventh degree or accidentally hits the wrong note; Mixolydian removes that conflict entirely.
When the accompaniment calls for Mixolydian
The Mixolydian mode naturally fits a one-chord vamp on a dominant seventh - a blues riff on E7, a funk part on A7 (see the article on targeting chord tones). If the accompaniment holds one dominant chord for a long stretch with no resolution, Mixolydian almost always sounds more confident than a plain major.
Hearing the difference: the same phrase, two sevenths
Play a phrase on the major pentatonic (which has no seventh at all - see the article on the pentatonic scale), then add the seventh degree, first natural, then flattened, listening for which one agrees with the dominant seventh chord in the accompaniment.
The fingering is already familiar
Like any mode (see the article on modes without seven boxes), there's no new fretboard position to learn - Mixolydian physically sits in the exact same fingering as a plain major scale, just with a different sense of tonic and one altered degree.
Try it in practice
Open chord trainer