The Mixolydian mode is a major scale used in a wide range of musical styles
including blues, bluegrass, jazz and Latin. It consists of the basic major scale
with the 7th note lowered a semitone. In the key of D this amounts to changing
the C# to a C natural. If we add this 7th to the D major chord, we have D7 -
commonly found in bluesy music. Compare this to the Dmaj7 chord arising from the
basic major scale, which is a much more relaxed, ambient sounding chord.
For Celtic music, the Mixolydian mode is extremely useful. Where the major
scale can sound a little square if used too often, the Mixolydian scale offers a
slightly darker, more aggressive sound which can add drive to a Celtic piece.
For example, this mode resembles the notes available on the bagpipes.
The D Mixolydian mode also provides the same fingering and chords for the G
major scale, which makes it especially useful to know.