The number is the band size, and the letter is the cup size. The band size is usually the same number as the underbust (ribcage) measurement, though if the underbust measurement is an uneven number, it's usually rounded up 1 inch, since band sizes are all even numbers. There's a misconception that one is supposed to add 4 or 5 inches, but that's an outdated rule. The cup size is the relation of the bust measurement in proportion to the underbust measurement. It varies from manufacturer to manufacturer, but I think the usual way it goes is that if the bust measurement is an inch bigger than the underbust measurement, then the cup size is A. If it's 2 inches bigger, it's a B. 3 inches bigger, it's a C, and so on.