The layer on the skin. It decides whether the rest of the system works or fails wet.
The warmth layer you add when you stop moving and the air drops.
The shield against the souly change and the wind chill. Only useful if you can vent it.
The layer that works whether or not you remember the sunscreen. Cover first, cream second.
Where the system usually fails first: wet socks, cold toes and a blister from the wrong boot.
The small kit that keeps the layers working past the first wash and the first snag.