Convergence means the capacity of the three RGB electron beams to strike the same point on the surface of the screen. Convergence is an essential factor because CRT monitors work on the principle of additive color. When the three colors are applied with the same intensity, they produce a white dot. Their absence results in a black dot. Varying the intensity of one or more beams thus creates colors. A convergence defect arises when one of the beams is not properly synchronized with the other two and shows up, for instance, as color shadows along the lines. This is mainly due to a weakness in the beam deflector or to incorrect alignment of the dots on the screen. It can also be affected by external magnetic influences.