Anyone who's tried to measure themselves for a bra has seen the "Add 5 for band" and wound up with a bra which has just too large a band.
When I wear forms, I don't usually attach them. However, I recently did, and found out WHY the band size differs.
When a bra is put on and pulled around back, it makes both breasts pull towards opposite sides. They come back when the bra is snapped, but think of how many times you've seen a woman do that, then adjust her twins.
However, when there are (non-attachable) breast forms, there is no pull; they just move on their own to fit any open area!
So if you are using non-attachable breast forms, the proper way to measure for bra size is to measure and then add 1 if an odd number, 2 if even, to give a little breathing room. The cup size stays the same as normal.
How did I find this out? I decided to attach a pair I don't normally attach, and my 'normal' bra suddenly didn't fit. And it all came together.
Sorry if this is obvious, but I've never seen anyone write on this; I know that Glamour Boutique has different sizing for men/women, and part of it may be the body structure....but most of the difference is just the boobs are attached on a woman and not usually on a man.
