If leeneia's dulcimer is a fretted mountain type, she'll be using modal reasoning all the time. With a dulcimer tuned to 2 sharps, you can play in:

D major
E dorian
G lydian/major hexatonic
F# minor/phrygian hexatonic
A mixolydian
B minor

all of which are common modes in Anglo-American folksong. You don't need to know you're doing this if you're playing solo, but for playing with others a common language really helps.