A deformation of a projected image—as from a slide or movie projector—that results when the lens of the projector and screen onto which the image is projected are not at right angles to each other. If the lens is directed upwards, for example, the projected image will be greaster in width at the top than at the bottom. Also known as the keystone effect, the term deriving from the trapezoidal shape of keystones used in architecture.

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