Having two babies, being put on steroids for health problems, weight gain and loss, and living at 3000 feet above sea level for 2 1/2 years which caused my lung capacity to increase and thus my ribcage to enlarge. This permanently changed the number part of my bra size (and probably saved my life the last time I had pneumonia and was bedridden, so it's all good). Cup size is a relative thing, dependent in part on that first number. The short version: The number part of it is a reference to the measurement around the rib cage directly below the breasts and the cup part tells you how many inches larger than that the measure around the fullest part of the breast is. An "A" cup is one inch. A "B" cup is 2 inches. Etc. Though the number part is a not a direct measure of the ribcage - ie a size 36 bra does not equal a measure of 36 inches around the ribcage. To make this arcane art of bra sizes even more unfathomable, they add a different number to the basic measure depending upon whether it is above or below a certain size -- and I no longer remember what the size cut-off is or what the different numbers are that they add.