I got my first Patagonia Union suit about 20 years ago, then it was made with heavyweight, stretch Capilene, and it’s lasted since then, seeing use on only the very coldest of days. The modern version is the new Capilene 4 One Piece, which though not as heavy as my vintage onesie will see a lot more use. The one-piece design keeps you warm by eliminating the separation between top and bottom where air and the other elements can get in, and are ideal when you’re going to be out in the cold all day. It also feels so smooth and comfy under your mid-layer that you’ll wonder how you did so long without one. A drop bottom allows you to tend to business, and a hood adds an extra layer of warmth up top. $199 www.patagonia.com