Watch the The "Celtic" Mode online guitar lesson by TrueFire from Play Acoustic Guitar 9: Advanced Style Variations
The "Celtic" Mode - Concept 10 is a video guitar lesson presented by Tony McManus and is sourced from The Celtic Journeyman.
I've talked about major scales and minor keys. There's one other very important mode in Celtic music that we need to be aware of. I'm going to illustrate this with a well known tune called "She Moved Through The Fair". Listen to this an ask yourself is it major or minor.
That's "She Moved Through The Fair"- beautiful song. It's a very poignant piece of music. Of course, it's a trick question- it's neither major nor minor. A good way of illustrating the mode I'm talking about –the name is Mixolydian- is to play that tune in a major scale. It sounds absolutely horrible. What I've done is to change all the Cs to C#. Now, this is in the key of D but it's not D minor as there's not a single F natural in the tune. But it's not D major either. It has a major 3rd but the 7th is flattened. Let me play the tune again....and there's the C natural, and again. All the Cs are C natural even though it's in the key of D- it's not D major nor D minor. It is D Mixolydian and there are modes associated with every degree in the scale. It's all very interesting and quite complex and all mostly irrelevant to Celtic music. One way to generate then is to look at a C major scale on the piano- the white notes from C to C. If you start on D you get a minor scale- that's the Dorian mode. If you start on F you get a different mode. If you start on G you get this mode- you get a scale with a flattened 7th. It's got nothing to do with the key of G, it's to do with the flattened 7th.
So why is that mode important and not the others? There's a physical reason for it and that's the Great Highland Bagpipe. it's precisely the scale of the highland pipes. The music is notated in the key of A though they play a semitone above that The music turns out in the key of Bb but it's exactly that scale with the flattened 7th and you find that mode all over Celtic music. So you have a major 3rd and flat 7th.