The difference between a t-shirt and an undershirt is that an undershirt doesn't need to look good. All an undershirt needs to do put a little fabric between your body and the shirt that matters. A t-shirt, however, must be outstanding on its own. It has to have a flattering fit, a comfortable cut, and be able to stand out on its own merits. And now that you've found yourself a t-shirt that can do all that, you'll never need a button-down again.