If the projector is not perpendicular to the screen then you will see the keystone effect where the image appears to be trapezoid shaped. Keystone corection allows you to correct for this problem by "squaring up" the image. The figure we show is the number of degrees out of perpendicular that the keystone correction can compensate for. Note that this will have a detrimental effect on the image so should only be used in the absence of other options.