Get fitted at some lingerie chains and you’re likely to hear the phrase “sister size” pop up. A sister size is a bra with the same cup dimensions. For example, the sister sizes of a 34C would be 32D and 36B.

On the one hand, this is a useful tidbit of bra size knowledge. Many women don’t realize that the cup size changes based on the size of the band, and this goes a long way in explaining bra size confusion. It illustrates, for example, why someone with a 30DD bra size might not look like a stereotypical “DD” and someone with a 42A won’t look like an “A” cup. (See Easy As A, B, C… The Bra Fitting Basics)

So what’s the down side of this cute little phrase? Simply put, if your band size is wrong then the bra DOES NOT FIT. The band is by far the most important part of any bra because it provides 90% of the support. Get a band that’s too big and all kinds of bra problems will pop up: the band will creep up during the day, the straps will create painful dents in your shoulders, and the girls will start to head south.

The bottom line on sister sizes? If you try on your regular size and it’s feeling a way too tight or loose, then it’s possible that that style does not run true to size. In that case, go ahead and try the sister size on for size. But if a store clerk is trying to convince you that the 38D will totally fit your 34DDD frame, be kind to your bust and walk away.