An aspect ratio is the ratio of width to height in a video display or image sensor. Traditional analog signals such as NTSC, PAL, D1 640×480, EIA, and CCIR video signals used a 4:3 aspect ratio (if the width of the image was divided into four equal units, the height of the image would be three of these units high). Naturally, this was also the aspect ratio of traditional displays and monitors, so that the content could fill the screen without being stretched.
With the introduction of High Definition (HD), televisions and monitors began being constructed with a wider 16:9 aspect ratio to match the new 1080p, 720p and 4K standards. It is now quite rare to find new displays that use the old 4:3 ratio. Most high definition surveillance cameras use this new 16:9 ratio as it allows for the image to fill the screen without distortion on these new monitors.