How do heavyweight base layers fit into a complete layering system?

With a heavyweight base layer, you can often run a simpler mid-layer system. In extreme cold, pair heavyweight base + light fleece mid + insulated shell jacket. In moderate cold with heavyweight base, you may skip the mid-layer entirely and go heavyweight base + insulated jacket. The key is accounting for the heavyweight base's extra warmth when choosing your other layers—don't stack a heavyweight base with a heavy mid-layer and a heavily insulated jacket, or you'll overheat even in extreme cold.