A contact binary is a highly evolved close binary star in which both components fill their Roche lobes. When both components have just reached the bounding equipotential surface they are sometimes described as forming a double contact binary. More often, both components exceed the equipotential surface and share a common convective envelope, as shown in the accompanying diagram. This configuration is sometimes as an overcontact binary. Further expansion would carry the envelope to the equipotential surface that contains the outer (L2) Lagrangian point, through which material woudl then be lost from the system.