Bra sizes in most countries have a number and a letter (for example, 34B). The number comes from the measurement of the ribcage directly underneath the breasts. The letter is a measure of how much larger the breasts are than the ribcage. This is found by measuring the chest over the breasts (across the nipples) and subtracting the ribcage measurement from the breast measurement, then finding the matching letter of the alphabet. For example, someone with a 3-inch difference between the breast measurement and the ribcage measurement would have a C-cup bra. However, this does not always work, since different manufacturers have differently-sized bras. Many women prefer to find a brand and size that works for them by trial and error.