I didn't know for years either and I'm still confused by it... I have a big rib cage and really small boobs. It's really hard to find a bra that fits, I usually need bra extenders. I actually have a plan to get a proper bra fitting soon, just need a couple hundred saved up to buy new bras. And I'm 32 years old... yes I am retarded when it comes to this. I had no mom to help me :(