The notes of the Mixolydian mode in tonic solfa are sung as “so, la, ti, do, re, mi, fa, so”. The G Mixolydian notes are G, A, B, C, D, E, F, G’, and you get this scale on a piano when you play upwards from G to G’ on the white keys. Again, note the distribution of the tone and semitone intervals in this scale:
The Mixolydian scale is the same as the Ionian scale except that its seventh note is flattened by a semitone. To convert an Ionian to a Mixolydian key signature add a flat to, or remove a sharp from, it. Likewise, to convert a Mixolydian to an Ionian key signature, remove a flat from, or add a sharp to, it. Many Celtic, Anglo-American and English folk melodies are Ionian/Mixolydian hybrids. There may be a seventh that is sometimes flattened and sometimes not. Or the tune may be hexatonic, with no seventh at all.
The Mixolydian scale is a major scale in that it has a major third. Where it differs from all of the 3 scales common in classical and art music (the Ionian major, the harmonic minor, and the melodic minor), and what it has in common with the Dorian and Aeolian minor scales, is that, in its ascent, it concludes with a tone rather than a semitone.
There are many well-known Celtic, Anglo-American and English folk songs that have a Mixolydian scale. Here is a selection of them.
All Around My Hat (GB/6a/1)
Tune Analysis: G Mixolydian, Heptatonic, Plagal.
Dabbling in the Dew Version 1 of 3 (GB/6a/62)
Tune Analysis: D Mixolydian, Heptatonic, Authentic.