Ideal for working on inlays or routing edges and dadoes, this sub base features a generously sized center hole measuring 2-1/2 in to accommodate larger bits. Unlike standard black bases, this model is made of sturdy, transparent plastic, so it won't block your view of your workspace. Existing screws from the router base also make for effortless, straightforward installation. The Porter-Cable 42188 sub base is compatible with Porter-Cable router models 100, 690, 9690, 691 and 693.