Abstract: A masking device is provided for use with projectors, such as an overhead projector, to selectively block the projection beam that carries an image disposed on a substrate, such as a transparency, for projection onto a viewing screen. The masking device has a framework having preferably guide rails that form slideways receiving a shutter panel. A transparency is aligned by the guide rails and the shutter panel has a leading edge that extends transversely across the aligned transparency so that only the image portion forward of the leading edge is projected. The shutter panel is constructed of a material that inhibits the projectiton beam, but allows the projection to read or view the image portion that is blocked. A base plate of transparent material supports the guide rails and provides a stage on which the transparency may be positioned for projection of the image disposed thereon.