Definitions for "Keystoning"
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.
The distortion of an uncorrected image projected on a wall or screen, typically shown to be wider across the top and shorter across the bottom, resembling a keystone of an arch.
Image distortion caused when the projector is not perpendicular to the screen, making the image noticeably wider at the top or bottom. Some projectors have keystoning correction functions. To correct keystoning manually, tilt the screen or the projector until the screen is perpendicular to the projectorÕs light beam axis. See also Focus Correction.
Shaping tesserae to maintain straight groutlines around curves.
Keywords:  facades, usage, building
Usage of keystones on building facades.