Newish Ginger tuned to "Gee, what exactly does that mean?"
General mountain dulcimer or music discussions
Dana, my previous post didn't really explain what a key is, and maybe that's what your initial question is.
Most songs center around a single note, a key tone (often referred to as the "tonic"). You can think of that as the home base of the song. Oh the song will meander to other notes, but it will always come back to that home base. But you can also play that song with a different key tone or a different home base. The meandering will move to different notes, but they will maintain the same relationships to each other and to the key tone. Think of that home base as the key. If you play a song with your home base on one note and I play the same song with a home base on a different note, we will sound OK individually, but we will clash when we play together. Each key comes with a set of notes that are likely to be used in that song and a set of chords that are likely to be used to accompany those notes.
When you use DAd tab to play while you are tuned GDg, it works because the relationships between the notes are the same. However, you will not actually be playing in the key of D (with D as your home base or key tone); you will be playing in the key of G (with G as your home base or key tone). So you will clash with dulcimers playing that same tab tuned DAd.
I hope that helps. If you want more, we should probably move to the Music Theory Group and start looking at the Circle of Fifths and other fun stuff that scares people unnecessarily.

Say it ain't so!