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Thermal underwear likely reminds you of the pajama-like long-sleeve cotton shirts and bottoms you may have worn as a child. While the function of long underwear remains the same—keeping your body warm and insulated—thermal underwear has come a long way since then. Merino wool and fully synthetic long underwear options provide thin but effective layers of insulation. Synthetics also provide heat-trapping and moisture-wicking qualities. You can layer thermal underwear under your clothes, but you can also wear thermal leggings alone for a stylish, form-fitting look.

Smartwool

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Tip: Consider the fabric. Merino wool is one of the best materials for thermal underwear. It’s warm, moisture-resistant, and much comfier than products made out of normal wool. It’s also lightweight, keeps its shape, and doesn’t retain odors.

BALEAF

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Tip: When you’re on the move in the winter, you need bottoms that are both flexible and warm. Thermal leggings are a great option, especially ones that are made with stretchy, synthetic fabrics. Thermal leggings are generally pretty thick and will trap heat against your legs without being see-through. The form-fitting look provides insulation—but still works as stand-alone pants.

Nooyme

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Tip: While leggings might be immediately what come to mind when talking about thermal underwear, you should also consider getting a base layer for your entire body. You can purchase a full-body thermal underwear suit, or you can mix-and-match thermal leggings with a thermal shirt. If you’re taking this approach, you should buy a similar shirt to match your leggings so you’ll be evenly insulated—and there’s no better way to accomplish that than by buying a matching set.

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