Aspect ratio is the width of a picture relative to its height. 4:3 and 16:9 are common aspect ratios. The majority of current NTSC transmissions are 4:3 aspect ratio. Virtually all HDTV transmissions are in 16:9 aspect ratio. Some HDTVs are 4:3 but display 16:9 programming in letterbox format (black borders on top and bottom of screen).