BlackHawk's Thermostatic Base-Layer Garments are designed to regulate body temperature, wick moisture, dry quickly, and control odor. The Engineered Fit series include long and short-sleeved tops, long bottoms, and boxer briefs; all allowing the wearer to choose between a body-contoured standard fit and a full compression fit. To achieve a compression fit, or a true next-to-the-skin fit, select a size one down from your normal size. The compression fit eliminates any potential friction related "hotspots" and helps to reduce muscle fatigue.