Letterboxing is what you see when a wide cinema film is shown on a screen that is a different shape — usually an older, squarer television. The whole cinema picture is kept, so nothing is cut off the sides, but black bands fill the top and bottom of the screen. The picture stays the same shape it had in the cinema; it is just smaller, with empty black space above and below. People still talk about it on phones and computers when the film is wider than the glass. The name is a picture: the film sits in a long thin window, like the slot in a letter box. There is a sister trick the other way round — black bands on the left and right — which technicians call pillarboxing. Letterboxing is a film-and-TV word; in ordinary talk people often just say “black bars” or “black bands”. The bands are on purpose, not a broken screen.

信箱模式（上下黑邊；為了維持電影寬銀幕比例）