aperture grille 
In some CRTs, the aperture grille is a series of wires stretched vertically down the inside of the monitor to mask the beams from the electron guns at the back of the tube. In other types of monitors, this function falls to a perforated metal plate called a shadow mask. Some users prefer the image quality of Sony Trinitron monitors, which use an aperture grille; others notice the faint horizontal shadows cast by the grille's tensioning wires and prefer other shadow-mask designs.