Tunings you like to use on your dulcimer
General mountain dulcimer or music discussions
If a group is playing instruments tuned to the keynote of D for example -- usually DAd -- then, technically you can play in any other "key of D" tuning (DAA, DAC, DAG, etc) and sound as if you are part of the group. You won't necessarily be playing the same notes as the others -- you'll often be creating your own "part" with notes that will blend with the others. But because you're all in a given keynote (say D) then it all can sound good together. I often play N&D style in the DAA or Ddd tuning while the group around me plays chords in DAd. I usually play the same notes as they do, but not on the same frets. If you're just beginning it is most helpful to you to play in the same tuning as the rest.
If everyone else is tuned to DAd and you're tuned to CGc or EBe it just isn't going to sound right. You'd all be playing the same frets, but not the same notes and it could sound rather bad!
Marg said "One more question, - if I can play DAd in all the (Mixolydian Modal Tunings)"
As I said above: You can use DAd tab to play ANY of the Mixolydian Modal Tunings: AEa, BFb, CGc, DAd, EBe, FCf, GDg
"...play DAd in all the (Mixolydian Modal Tunings) is a non sequiteur
You can't play DAd if others are tuned to GCc, or vice versa. Trust it will sound "not good"!