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.
Other articles related to "letterboxing":
... Pillarboxing (reversed letterboxing is the display of an image within a wider image frame by adding lateral mattes (vertical bars at the sides) for example, a 1.331 image has lateral mattes when displayed ...