Abstract: An accessory for use with an overhead projector comprises a platen which has a generally rectangular, transparent central portion and an opaque peripheral portion bounding the central portion. In the region of peripheral portion the platen has two mutually orthogonal, upstanding ridges, each extending substantially the entire length of the corresponding side of the central portion and each being defined by a pair of opposed inner and outer ridge-defining surfaces. The inner ridge-defining surfaces of the two ridges serve to locate a plurality of superimposed transparencies in register with each other and with respect to the central portion. The outer ridge-defining surfaces of the two ridges each form a guide surface for guiding a highlighting strip, or an occlusion or revelation screen.