Tara wrote on another thread recently ( hope you don't mind my quoting it?)
"Your bra size - 36DD - consists of the BAND size (36) which is the measurement just underneath the bust with either 4" or 5" added to get to the proper band size, and the CUP size (DD), which is the difference between band size and cup size.
(This is just one method. There are others.)
http://www.herroom.com/bra-fit.....01,30.html "
I had a look at the link and can now say that I have an underbust of 29" (having expelled the air out of the chest, as per instructions) and a bust of about 34 inches which should mean I am an AA cup. While I do have some AA cup bras (mostly hand me downs from my Mum I also have A and B cups and don't consider myself to be Trinny (of T & S) like-flat. I'll probably get flatter soon though. Anyway I was just surprised that the bra sizes started at a difference of over 4 inches. I suppose there are options in the junior departments.
BTW is 29" quite a big underbust measurement? I have always thought I had a big rib cage, though I don't carry any fat or muscle bulk in that area, and that measurement would tend to confirm it