system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why a widescreen movie played on a widescreen TV still has the thick bars on the top and bottom?
gpt
Because some widescreen movies have pictures that are still wider than HDTV.  HDTV has a picture that is about 1.78 times wide as it is tall (this is called the "aspect ratio", expressed as 1.78:1 or 16:9 because 16 divided by 9 is roughly equal to 1.78).  Many movies have a "Flat" aspect ratio of 1.85:1, as the picture is only slightly wider than HDTV, such movies can play on HDTV without needing black bars.

But some movies are much wider.  Many movies have an aspect ratio of 2.35:1, also known as "scope," in memory of the first widely used wide-screen presentation format, 20th Century Fox's "Cinemascope."  Some movies are wider still.  The wider moves still need black bars top and bottom on an HDTV in order to show you the full width of the picture.

And no, there is no picture information hidden by the black bars.