Why are some bras 2 to 3 letters in measurements?

Why are some bras 2 to 3 letters in measurements?

my mom was a female underwear/lingerie retailer when I was a kid and I was always curious about the measurements bra use.

thanks to my mom's customers I learn that the number represents the measurement of the upper body; clearly no different than pants while the letter represents the actual size and volume of the woman's boobs. obviously the higher the letter is the bigger the boobs are (totally the opposite of a report card if you ask me)

till then at first I always thought they when from A, B, C, D etc but then I started realizing that there are bras that are D, DD, and DDDs etc. so I was wondering wth does it mean when a bra has more than one letter. does this mean that DDs are the same as E, if so then what does DDD equal to? or does this mean that DDs are "almost" as big as Es but not quite as big? making DDD slightly closer to E?

Why are some bras 2 to 3 letters in measurements?
Why are some bras 2 to 3 letters in measurements?
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