Letterboxing is the practice of transferring film shot in a widescreen aspect ratio to standard-width video formats while preserving the film's original aspect ratio. The resulting videographic image has mattes (black bars) above and below it; these mattes are part of the image (i.e., of each frame of the video signal). LTBX is the identifying abbreviation for films and images so formatted. The term refers to the shape of a letter box, a slot in a wall or door through which mail is delivered, being rectangular and wider than it is high.

Letterboxing is used as an alternative to a full-screen, pan-and-scan transfer of a widescreen film image to videotape or videodisc. In pan-and-scan transfers, the original image is cropped to the narrower aspect ratio of the destination format, usually the 1.33:1 (4:3) ratio of the standard television screen, whereas letterboxing preserves the film's original image composition as seen in the cinema. Letterboxing was developed for use in 4:3 television displays before widescreen television screens were available, but it is also necessary to represent on a 16:9 widescreen display the unaltered original composition of a film with a wider aspect ratio, such as Panavision's 2.35:1 ratio.