Guest, leeneia wrote:
"I just sat here at my piano keyboard and sounded out The Great Silkie and She Moved through the Fair, choosing a D as my starting note. Both tunes start and end on the note D. The only black note used is an F#. Therefore, they are in the key of G, and they are in the Mixolydian mode, the mode based on the fifth note of the scale. Although the songs would be accompanied by the chord D, the key of D has nothing to do with them, because they don't use C#, a note that D never leaves home without."
She's right about the songs being in the Mixolydian mode, but wrong about the key. If D is the hometone, then D is the key. However, the SCALE is mixolydian, so there is no C#. A mode is a scale -- a pattern of whole & half steps, nothing more, nothing less. D Major has two sharps (F# & C#), D minor has 1 flat (Bb), D mixolydian has one sharp (F#), etc.
With the folk songs that have a home tone at odds with the key signature, the trick is to look at the pattern of steps. In addition to the Mixolydian mode (which looks like its missing one accidental), minor-y sounding trad songs alternate between the Dorian and Aeolian. If it looks like it's in the right key, then its probably Aeolian, if not, it's probably Dorian. Take up the dulcimer, and you'll soon have your modes down pat.