Aspect ratio is the ratio of image width to image height regardless of the size of the image. An Aspect Ratio is expressed with two numbers separated by a colon. The number broken up into two parts X:Y where the first number represents width and the second number represents height. Example: The following 3 screen sizes all have the same aspect ratio of 16:9.
16 inches wide by 9 inches high
16 centimeters wide by 9 centimeters high
8 yards wide by 4.5 yards high
Even though the measurements are all different, they all share the same ratios of 16:9. The most common aspect ratios used today in Movie Theaters are 1.85:1 and 2.39:1. The most common aspect ratios used today in Home Theaters is 16:9 because this is the standard format for HD Television and widescreen. 4:3 is still a very common and standard aspect ratio used by most Television sets and Computer Monitors.