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Your widescreen TV has an aspect ratio of 16:9 (1.78:1), which is the aspect ratio of HDTV.
However, movies are filmed at several different aspect ratios, including 2.35:1. Also referred to as "CinemaScope", 2.35:1 is a very panoramic aspect ratio that provides a wide field of view in the theater.
"CinemaScope" is much wider than your widescreen TV, so it still requires the use of letter boxing ("black bars") to fit the entire image on screen. Some TVs and DVD players have a zoom function that allows you to blow up the image so it fills the screen, but the sides of the image must be cropped in order to do so.