High-Def Digest has a really good simplified explanation about different movie aspect-ratio and what happens when the films make the leap to home-video with different size screens. It talks about pan-and-scan, letterboxing, pillarboxing, and also open-matte, wherein you actually get more of the image that was intended for theatrical presentation.
As a curious completionist (read: pack-rat) I tend to like the idea of having an open-matte movie because you get to see extra goodies, but I understand fully well its consequences.
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