You’ve probably heard of sister sizing in the lingerie, for many women they wear a bra band that is too big and cup too small. Many think that a DD cup is bigger than a C cup and in some cases that’s correct but unless you know the back size as well , a bra measuring a C cup can be bigger than a DD cup.
This is where sister sizing comes into play. Before we get into that, let’s talk about what the numbers and letters represent. The numbers (from the UK measurements) represent the measurement taken from under the bust. And the letters represent the difference between your back and bust measurement. Each inch you breasts project from your ribcage is a cup size. For example a 36C, 34D and 32DD are all the same cup size, but a 34D breasts project out 1 inch less than a 32DD but because the different from the band to the cup has increased they are both the same cup size.
So a woman wearing a 36C can wear a 34D , and how it would fit, it would be tighter around the ribcage but the cup would be the same. So technically a 38C cup wearer has smaller breasts than a 30DD cup wearer.
SO is a 34D really the same as a 32DD?
Fundamentally yes. The same wire is used (and for a 38B, 36C & 30E, and if you were using a moulded cup then that would be the same size. The wings on the 32DD would be shorter than a 34D, hence why if your under-band of your bra is too big when wearing a 34D, you can wear a 32DD for a better fit.
Now this sister sizing doesn’t work on all sizes, when you reach above a E cup, tweaks have to be made for example a 34F and 32FF may have a slightly different neckline measurement, as with a 32FF there is usually more boob than back, and to keep them projecting forward and not under the arm pits, the apex on each cup may be closer together than a 34F. Although the wire might be the same so the manufacturer doesn’t have to produce a wire for every size bra.
If you are designing a collection, it may be worth taking sister sizing into account when deciding on your range, as the bra wire minimums are going to be the same whether you’re producing for a 32DD so using them for a 34D makes sense to have that in the range as well.
If you are grading your lingerie, I always start with the cups, because they are the main importance and when you grade a 32DD, you’ve graded the 34D, 36C etc as well, therefore it’s then easier to work out which wings need to be graded as well. I have always worked for companies that have used the sister sizing, so it’s a useful bit of knowledge to know which cups correspond with another, for both fit and ordering components and wires.
When ordering components even though the 34C is technically the same cup size as the 30DD, different elastic may be used. A 30DD wearer may need wider elastic for the bra to hug and support the weight of the breast, whereas the 34C has a wider distribution for the weight of the breasts along the under-band so a thinner elastic may be used.