If the image looks like or , the projector has been placed off to one side of the screen and angled toward it. Use the horizontal keystone slider to correct the image. If more correction is needed, move the projector so that it is nearly centered with the screen, then use the horizontal keystone slider to adjust the image shape.
If the image looks like or , make sure the projector is not tilted up or down. See Positioning the Image.
Make sure the Auto-V keystone is turned on.
Note: For best image quality place the projector directly in front of the screen and minimize the need for horizontal keystone correction.