Squeezebox StudioKeys and scales for your box
A two-row box plays some keys easily, some with a stretch and some hardly at all. These pages check every key against the actual buttons, then show where each scale lies.
What keys can a D/G melodeon play in?
The usual English melodeon: a D row outside and a G row inside, with the home notes on button 3.
What keys can a G/C melodeon play in?
A fifth lower than a D/G, with a G row outside and a C row inside: the standard Hohner layout.
What keys can a B/C button accordion play in?
The Irish box. Two rows a semitone apart, B outside and C inside, so nearly every note is there somewhere.