The
pillarbox effect occurs in
widescreen video displays when black bars (
mattes or masking) are placed on the sides of the image. It becomes necessary when film or video that was not originally designed for widescreen is shown on a widescreen display, or a narrower widescreen image is displayed within a wider
aspect ratio, such as a 16:9 image in a 2.39:1 frame (common in cinemas). The original material is shrunk and placed in the middle of the widescreen frame.