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 1.85:1 image in a 2.35:1 frame. The original material is shrunk and placed in the middle of the widescreen frame. Some older arcade games that had a tall vertical and short horizontal are displayed in pillarbox even on 4:3 televisions.
Pillar boxing is the vertical equivalent of letterboxing and is sometimes called reverse letterboxing. Its name is derived from its resemblance to pillar box-style mailboxes used in the UK and the Commonwealth of Nations. The four-direction equivalent is called windowboxing. Likewise, the vertical equivalent of pan and scan is called "tilt and scan" or "reverse pan and scan."