Originally Posted by **********

They are unrelated.

4:3 and 16:9 are aspect ratios (width:height). SD and HD refers to the resolution, lines or pixels. You can still have High Definition video in 4:3, though its not popular.

Saying "most SD footage is being shown in 16:9 on televisions rather than 4:3" is not accurate because you're kind of mixing apples and oranges.

Different TV shows for example are presented in different ways. For example: 4:3 content can be "stretched" to 16:9 but it looks awful, making people look short and fat.

Some news channels have a cool trick. They show 4:3 video of a news report, and add a kind of stretched background to fill up the remaining space, made up of a blurred version of the original content. Kind of a smart trick.

The best are TV shows such as Seinfeld and Star Trek TOS. Both TV shows were originally shot on film, in 16:9, but "framed" for 4:3. Today they are shown in spectacular HD and 16:9 format because they took the original film, re-framed it without having to zoom in to fill the space except for a few rare occasions.