A cover slip, coverslip or cover glass is a thin flat piece of transparent material, usually square or rectangular, about 20 mm (4/5 in) wide and a fraction of a millimetre thick, that is placed over objects for viewing with a microscope. The object is usually held between the cover slip and a somewhat thicker microscope slide, which rests on the microscope's stage or slide holder and provides the physical support for the object and slip.