Letterboxing (filming)
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.
Read more about Letterboxing (filming): Description, Early Use of The Letterbox Format, In The Cinema and Home Video, On Television, Pillarboxing and Windowboxing