Individual modular stair units come together as a double helix in Linder Enterprises' new engineering process for circular stairs. The swirling units can be completed with wrought iron, steel, wood, or glass railings. Spec the systems inside or out based on cladding choices of wood, metal, plaster, or polyester resin. Linder Enterprises, 507.553.3040; www.linderenterprises.com.