Certain television formats such as the 16:9 format or film formats such as the cinema widescreen format or Cinemascope with 21:9 can only be displayed with restrictions on standard screens with an aspect ratio of 4:3.
If these formats are adapted to the width of the 4:3 screen, wide black bars appear at the top and bottom and the actual display appears as a narrow strip, as if you were looking through a letterbox slot. Hence the name letterbox.
The size of the upper and lower black bars is calculated from the number of lines displayed. 576 lines are visible in a 4:3 format with 625 lines. With a 16:9 video signal, only 432 lines are displayed on the 4:3 screen, and with 21:9 films, only 326 lines are displayed.widescreen image in the 16:9 format or cinema films in the cinema formats of 1.67:1 and 2.35:1 - corresponding to an aspect ratio of 21:9 - so that they can be displayed on the 4:3 full format. The image is stretched vertically in this display, the number of displayable lines is reduced and the image sharpness deteriorates.
Letterbox is used in DVD drives, among other things, to adjust the format of the various film and television formats.