Fitted layer that sits directly under your kit to regulate temperature and manage sweat. Material wicks moisture away from skin so you do not feel damp and uncomfortable during intense effort. Stretches with your movement rather than restricting anything. Keeps you warm in cool conditions without adding bulk. Does not bunch up or shift around as you move. Sits flat under your top so it does not create visible lumps. Quality construction means it lasts through repeated washing and the elastic does not lose shape. These are the base layers that work during training rather than just feeling restrictive.