This term is used to describe a result which arises when the film plane in a projector or camera is not parallel to the screen or view. The perspective distortion that follows this produces a border to the picture which is in a trapezoidal shape, which resembles the ‘keystone’ of a masonry arch. When talking about stereo, the term is applied to the projection or capturing of two images where the cameras/projectors are toed-in so that the main objects coincide when viewed. The scenes proportions then have slight differences that produce some mismatching of the borders/outlines on both images. To produce keystoning in depth (which is more correctly known as the frustrum effect) a gross departure from the orthostereoscopic practice needs to occur when using telephoto lenses.