Owners of broad root breasts have breasts whose base is bigger than their depth and goes far to the sides of their chest. Their breast tissue has little projection forward and stays close to the chest. The broad root breast is therefore a breast form - it is especially common in smaller cup sizes, but it can occur in any size.
Broad root breasts need very wide wires and shallow cups. Since this breast form is very common in smaller breasts and a lot of people don't know where their breast root is, a lot of women wear bras which are much too small for them in the cup and the wires. (At least before they got to know the wondrous world of bra fitting! :))
The breast tissue can appear hidden in broad root breasts and so the owners may think that anything above a B-cup will definitely be too big for them (this is a form of letterphobia.) But bra fitting puts almost everyone in bigger cups than they're used to - including owners of broad root breasts. After all, it's important to have bras of which the cups work for your breasts and not the other way round.
To find out if you can call yourself a proud owner of broad root breasts, look at the distribution of your breast tissue. Often, your breasts will look bigger from the front than from the side. Since the tissue is very flatly distributed, there won't be big curves visible in the profile of your torso. The breast tissue is distributed over a relatively big part of your torso when compared to other breasts.
Both conditions have to be met for your breasts to be broad root breasts: They're
broad and therefore cover a relatively large area on your torso and
they're shallow, therefore don't have a lot of projection forward.
If you identify your breasts as broad root breasts, please also read our article about the orange in a glass, since it's a common fit problem. It's really important to have well-fitting wires that enclose all of your breast tissue!
Resources and further reading
On Kurvendiskussionen: Bras for smaller busts, click the flag symbol on the upper right for English text. With model recommendations.