Many flat-screen TVs can only show HDMI signals in Wide (16:9) mode, which means that material recorded in 4:3 aspect ratio will be shown incorrectly stretched when shown via the HDMI output.

Some recorders can squeeze a 4:3 image so that when the TV stretches it out again, it will have the correct aspect ratio on the TV, but with black bars at the sides. This is called PillarBoxing (like LetterBoxing, but on the sides instead of top/bottom).