Perforated grille used in cathode ray tubes to ensure electron beams hit only the phosphors they are intended to excite at a given time. To illustrate how this works, punch a small hole in a sheet of paper and hold the paper up fairly close to a light bulb or flashlight. The beams of light being emitted from the bulb spread out. If you want them to hit a very specific point, they must be directed. The deflection yoke directs the electron beam in a picture tube. However, some electrons spray out to the sides away from the beam, dispersing themselves. The shadow mask stops these stray electrons from hitting phosphors other than those intended to be stimulated by blocking them. Holding your paper up to the light, you will see a small point of light emerge through the hole to hit a specific point while the other light simply reflects off the paper. A shadow masks works in the same way with electrons passing through small holes to hit the target phosphors and stray electrons simply hitting the shadow mask, just like how the light hits the paper and allows only a small, focused beam to pass through.