Aspect ratio is the way movie directors specify the actual size of the image that will be projected in the theater, as well as in the home. These ratios often confuse the heck out of consumers and the numbers can often be unwieldy. Traditional standard-definition television sets are built to operate at a 4:3 aspect ratio, which is much squarer than the 16:9 aspect ratio that is commonly found in HDTV broadcasts.
The motion picture industry puts a value of 1.0 to the video image's height. Therefore, an "anamorphic" frame is described as 2.40:1 or "2.40." In American movies, the common projection ratios are 1.85:1 and 2.40:1. CinemaScope, also known as 2.35:1, is another popular format used in filmmaking. Digital cinema uses 2.4:1 aspect ratio. Other parts of the world use a slightly squarer or 16:9-like aspect ratio.
Nearly all LCD and plasma HDTVs come in the 16:9 aspect ratio, which means 2.35:1 films will often black bars on the top and bottom. Front-projector home theaters at the ultimate level often have motorized "masking" screens that move physical elements to the exact size needed by the source material, while absorbing any unwanted bleed of light from the sides of the screens. Auto-masking screens are about as cool as it gets in the world of "wow" factor.
Anamorphic lenses are quite popular in home cinemas these days. These lenses, when used in conjunction with a wide screen and a projector with the necessary processing, allow for 2.35:1 movies to be projected using the entire 16x9 chip (or chips) of the projector.