Distortion of a projected image on a screen, where the image is wider on top and narrower on bottom. The image distorts from a rectangle into a trapezoid because the projector is above or below the center point of the screen; either the top or the bottom become wider because it is further away from the lens. To adjust for keystoning, the top of the screen can be tilted a little, either forward or back. Many data projectors allow you to tilt the lens to solve the problem.