Convergence
Color CRTs rely on three separate electrical beams to project simultaneous red, green, and blue images; these combine to form a full-color image. If these beams are not precisely aligned, the red, green, and blue portions of the image may not overlap correctly; the image will look unfocused and will have visible colors along the edges. When the three beams converge correctly at all points on the display, you get a perfect image.