Lightweight tees, whether fitted or not, are easier to tuck into high-waisted bottoms without causing any odd bulking. Weight matters for other reasons, however: The heavier a shirt is, the more it’s designed to wear casually over tighter bottoms.
On the other end of the spectrum, Everlane’s organic cotton tee may be made from more heavyweight stuff, but it’s still very much suitable for mid-season dressing. Of course, a T-shirt made from a heavier fabric will tend to keep you warmer, while a lighter fabric will tend to keep you cooler, but that’s not necessarily the case: For example, our thermal pick from Uniqlo is made from fairly thin fabric, but is designed to warm you up.