This site has great photos showing the difference between wearing the wrong bra and the right bra. The other day, a fit-tester told me she wore a 34DD, but when she also told me that she shopped at Victoria’s Secret, I had to take her to Linda’s. She turned out to be a 32G.
That difference was pretty obvious, but I have difficulty identifying more subtle differences. This is why I cannot recommend expert bra fitters enough. Once you know your size in a specific brand, you can often purchase it online. But at least purchase the first one from your bra fitter. She provides a major service. (Nothing more after the jump.)