I Need a Word

I'm trying to remember a word or possibly a phrase and I can't for the life of me dig it up out my brain right now.  It basically means two halves of a whole, something like Yin and Yang, something like you must have both parts for it to be right, to operate properly, to function best.  Something like balance.  It's driving me crazy.  Help.

I Need a Word

I'm trying to remember a word or possibly a phrase and I can't for the life of me dig it up out my brain right now.  It basically means two halves of a whole, something like Yin and Yang, something like you must have both parts for it to be right, to operate properly, to function best.  Something like balance.  It's driving me crazy.  Help.