An apparatus and a method to hold a patterning device configured to impart a beam of radiation with a pattern in its cross-section. The apparatus includes a base configured to support the patterning device and an inner cover couplable to the base. The inner cover includes a restraining mechanism that, upon an application of a force external to the inner cover, is configured to provide an in-plane force to the patterning device to restrain movement of the patterning device, the in-plane force being substantially parallel to a patterning surface of the patterning device.