We should add that an optical lens shift will not cause the kinds of distortions that are caused by digital methods as described in the last post. Shifting the lens changes the image in such a way that it anticipates different distances for the top and the bottom (say) giving a correct final result. Some projectors have both horizontal and vertical lens shifting, allowing the projector to be off-axis horizonally, vertically, or both.
Lastly, many projectors are designed to be off-axis. Such a projector is intended to be aiming from the bottom (or dually from the top) of the screen. The term "projection angle" refers to the angle anticipated by such a projector. A bit of trigonometry allows you to compute where to situate the projector for minimal keystoning. Obviously, if the lens is a zoom lens and can therefore change the focal length, this also has an affect on the situation.