We all wear them (well, most of the time, anyways), and yet even for the small-chested, bras can be the most frustrating garment. Often, they’re the first thing we want to fling off the second we walk into the house (and I’m not talking in a sexy way—more like a must-get-comfy-this-instant kind of way) and the thing we ruefully put on in the morning as we get dressed for work. They may even get a worse rap than pants, widely considered Enemy No. 1 of the millennial generation (want proof? BuzzFeed’s anti-pants posts have collectively racked up more than a million views.)

But, bras don’t have to be the absolute worst. In fact, they can be your friend, so long as you buy ones that are comfortable, supportive, well-fitting and fun to wear. Whether you go in for a professional fitting, do it yourself at home or opt for a more holistic method, like True & Co.’s online fit quiz, you’re always better off when you come equipped with a little knowledge of your own about what measurements mean, what shape your breasts are and why different bras are better suited to certain body types.

To that end, I’ve enlisted the help of Aerie's Fit and Style expert Jenny Altman (whose career just so happens to include a stint as a former Lucky fashion editor) and True & Co. founder Michelle Lam, whose data-based approach to bra retailing has enticed more than one million women to take their signature fit quiz, culminating in the brand new shape-based fit system TrueSpectrum.

As it turns out, it’s not just customers who struggle with the complexity of bra shopping. “Bras are the most complicated garment you could possibly make,” admits Lam. “There are 20 components in a bra. There are only six components, by the way, in a pair of jeans, which is another ‘hard to fit’, complicated item. With a bra, it’s incredibly hard to diagnose the right fit, because there are so many different dimensions—and second of all, to find a bra that will actually do the job; that will actually fit and also look beautiful at the same time.”

Here, with Altman and Lam’s assistance, as well as the collective wisdom of some devoted online communities, we’ve broken down a few of the need-to-know aspects of bra fit and sizing:

BAND SIZE: Simply put, the band size correlates to the measurement around your ribcage underneath your breasts—but there is some dispute about exactly how they relate. Namely: the “add-four” method. Many major department stores and American retailers take the ribcage measurement and add four inches (or five if it’s an odd-number) to obtain the band size. Online, however, communities like the subreddit r/ABraThatFits are strongly against the practice, and encourage trying bras with band sizes that match your ribcage measurement exactly (even if this means you’ll wear a much bigger cup size than you’re used to.) Since most of your support should come from your band (and not from your straps, because ouch), it’s important to find a size that is comfortable enough to wear all day, yet isn’t so loose that it doesn’t do the job. Lam agrees that support is important, but also says that there is a degree of individual preference that has to be taken into account. Many American women may be used to wearing a looser band, she says, and so buying a smaller size (or even buying the same size when you’ve become accustomed to a stretched-out standby) may be an uncomfortable adjustment. “Some women are used to a much looser band, and if you’re an A-cup or a B-cup, it really doesn’t matter,” she says. “If you’re a D-cup or above, it does, and we’ll help you give you more education to figure out which one it the right one for you.”