Umm. Simple answer: because the notes in the book say so? If memory serves, Dorian is like playing all the piano white keys from D to (shining) D. I have a Jean Ritchie (?) dulcimer book at home with all the modes explained like that ... but it isn't here at the office. I think there are quite a few Welsh tunes in the Dorian and a few mixolodian. I'm sure I've got an article on it somewhere ... probably one of the Welsh Folk Song Socs. journals...

sian