Like denim, good t-shirts should be the foundation of any girls' closet. As firm believers in effortless dressing, we think the right tee can make every outfit cooler and every trend more manageable. That said, here are the four styles you should start with, but feel free to add more! Must-Have Tee #1: Slim, Snug, and Structured The key to this t-shirt is buying one that's true to size and fitted through the body and arms. Look for a crewneck cut and crisp, clean colors. This tee should not drape or bag, as it looks best paired with more voluminous pieces, like a wide-leg trouser or knee-length full skirt. Must-Have Tee #2: Relaxed and Rolled This tee should be slightly boxier in proportion and looser through the neck; it should show off a little collarbone. We picture this one rolled at the sleeve and loosely tucked into a tight, skinny, ankle-length jean, a fitted skirt, or a dressy short. Must-Have Tee #3: Graphic and Gorgeous We love a tee--both fitted and relaxed--that boasts a clean, graphic element. Whether it's a bold stripe across the chest or a message (think: the Who What Wear Love Tee!), these shirts look amazing with anything from a printed trouser to a rich leather pant. Must-Buy Tee #4: Rock 'N' Roll and Worn At some point, you're going to want to celebrate your rock 'n' roll side, and these occasions call for a perfectly faded tee. A faded concert t-shirt is always a safe bet--the more worn-in, the better--so keep that in mind when you're scouring flea markets, eBay, or thrift stores. We love this type of tee juxtaposed with something very girly or polished, like a metallic pencil skirt or a slim, cropped suit.