Aperture grille

An aperture grille (tension mask) is one of two major technologies used to manufacture color cathode ray tube (CRT) televisions and computer displays; the other is shadow mask.

Fine vertical wires behind the front glass of the display screen separate the different colors of phosphors into strips. These wires are positioned such that an electron beam from one of three guns at the rear of the tube is only able to strike phosphors of the appropriate color. That is, the blue electron gun will strike blue phosphors, but will find a wire blocks the path to red and green phosphors.