Cerium Overlays are coloured sheets of robust transparent plastic, developed by CVT. 12 different colours are available. The overlays are UK paper size A4 that approximates to 297 x 210 mm or 11.7 x 8.3 inches. Overlays can easily be cut in half to use with smaller pages. Coloured overlays in A4 size can be preferable to the smaller, reading rulers in a number of cases.