A C box is the most popular for Cajun music, but what key for a second box? B flat for Zydeco, or a D?
What?

Key of D would be the next most useful, not to mention playable. The reed response on a D is really nice.

D and Bb are popular choices. but something you might want to consider: do you sing? What keys are you most comfortable with? An accordion in a key that's compatable with your vocal range inspires the creative juices and is a joy to play. For me, C is not always an easy key to sing, no matter how much I stand on my tippy toes, it ain't happening. I put on the A box, and for some songs, all is right with the world. Since you all ready have a C box, you've got a few options open when thinking about a second accordion.
Steve

Bb.
Might as well go with something a little further away from the C.
D is only one key up from the C.

Plus, with the Bb, yuh can play along with some early Beau Jocque, Boozoo and Zydeco Force so yuh can work on those Creole grooves!

G-G-G-G-Git it!

R!CK

Gotta love that Bb (it's better than rocky road ice cream)... but remember, it's not just for Zydeco ... a gob of traditional comes out'a that Bb key in these times.

If ya got a twilly squeeky little D singing voice and can do it without popping a vein in your tonsils, get the D box... otherwise that grunt n' goan sound of Bb is hard to pass up in my estimation.

I hadn't thought of the singing question. I have played the harmonica for a good while and Bd is my favorite key in that. More minor notes and pitched lower, which may make blues much easier (another of my interests) Many good reasons for each choice, it's a personal thing. Right now the Bd seems to be in the lead.

I say D and then Bb after. Here are some other factors to consider. Where do you live? Who do you think you might be jamming with? Most fiddle players are tuned standard (GDAE), not tuned down one tone (FCGD)like in SW Louisiana. It is much easier for fiddler to jam with you if you're playing a key of D box. For a key of C box, many fiddlers will prefer to tune down to capture the effect of the drone strings which make the cajun sound unique. From what I know, the Bb box is more typically associated with Blues and Zydeco music. Again, you singing tone should also be a factor.
Maz

Rick Reid said: "Bb. Might as well go with something a little further away from the C. D is only one key up from the C."

Uh, unless I'm missing something, Bb and D are exactly the same distance from C, namely two semitones: Bb - B - C or C - C# - D, it's the same.

Here in Austin there are not that many Cajun musicians. Zydeco is much more common. Of course, there are a whole mess of Bluegrass/Irish/Folk. Maybe I should just get a three row. No hurry for now. I am doing fine with my C box

Keystones were kickin' in. You are correct, sir.

R!CK