These zero order waveplates are made from two pieces of quartz, either cemented or optically contacted, with their optic axes perpendicular to each other. Their thicknesses are slightly different in order to provide a net zero order retardation. They can be used to change the state of polarization of incident light. A quarter wave plate has a net retardation of pi/2 and changes polarization from linear to circular or circular to linear. A half wave plate has a net retardation of pi and rotates the plane of polarization by 90deg. These waveplates are available either unmounted or mounted in a metal ring.