In a scroll compressor, means for maintaining the motion of an orbiting plate in fixed angular relationship to a stationary plate and for modulating the capacity of the compressor. The orbiting and stationary plates include intermeshed wrap elements of similar spiral shape about an axis. A coupling ring is slideably disposed between the plates and sealingly encloses the wrap elements. Four slots are provided in the coupling ring, aligned at right angles to each other and spaced apart around its inner circumference. Three segments, extending outward from the wrap elements, sealingly engage these slots. The fourth slot is engaged by a sliding key attached to one of the plates. The segments separate a first and a second fluid inlet and cooperate with the slots in the coupling ring to constrain the orbiting plate's motion in fixed angular relationship to the stationary plate. Valves on the first and second inlets are provided to control the flow of fluid to be compressed into the separate inlet volumes defined by the segments, thereby modulating the compressor's capacity.