Long johns, base layers, skivvies—there are many names for thermal underwear for men. But the main purpose of these cozy tights is the same no matter what you call them: they retain heat and wick sweat away from your skin, keeping you dry and warm.

Classic thermal styles only employ cotton knitted into a waffle pattern for warmth, but materials like wool, nylon, and polyester have become popular options because of their durability and insulating properties. Some pairs of thermal underwear for men are suitable for cold-weather activities like snow-shoeing or riding a fat-tire bike; others prioritize comfort for lounging après ski; and a select few do both.

In some product descriptions for thermal underwear, the brand will list the garment’s suitability for high output or mixed output. Output refers to your activity level.

If you want something that’ll keep you dry while you’re working up a sweat cross-country skiing, opt for a lightweight, high-output pair. A thin profile can help the long johns move moisture away from your skin more quickly instead of a thick pair that’ll get weighed down when it gets damp.

Say you prefer to bundle up while you lounge by the fire with a book and a cup of tea, or you need something warm to sleep in during the winter—a thick pair made for low or mixed output will do the trick.