>TV shows (older ones) are often 4:3 aspect ratio. Some video players offer the option to play them "full screen" regardless of the monitor's aspect ratio, so that can force a 4:3 video to play 16:9 on a widescreen monitor. In that case yes, the picture would appear stretched horizontally.

Thanks for the link, now I finally understand what did they mean with "letterbox", because there doesn't seem to be any lettering involved. Now I understand that whichever moron devised the name deserves my respect. Such a stupid idea doesn't come by itself, one has to work hard and long to come to it.

And even the original letterbox is not a box at all.

There's no logic. There are no rules. There are lists and individual histories for each word.