18.10Mixolydian

Take G using C-major notes:

G A B C D E F G

Compare G major:

G A B C D E F♯

Mixolydian lowers the 7th:

F♯ → F

Formula:

1 2 3 4 5 6 ♭7

This matches the structure of a dominant seventh chord:

G B D F

So Mixolydian naturally fits a non-altered dominant chord.