For example, most older television sets in the United States have a ratio of 4:3 (1.33:1, known as "Academy Ratio"), meaning that the image you see is four units wide and three units tall. Widescreen televisions use 16:9 (1.78:1), but this format didn't really catch on in the USA until High Definition television broadcasts caught on. Interestingly, most (but not all) High Definition cameras used for film shoot at a 16:9 ratio.
The most common aspect ratios for feature films today are 1.85:1 (sometimes mistakenly called "16:9"; generally used for comedy, drama, or other small scale projects without much emphasis on set design or scenery) or 2.39:1 (also known as CinemaScope or "Scope"; generally used for action, science fiction, "epic scope", and other large scale projects). Most older films used the 4:3 aspect ratio; the "Academy" in Academy Ratio is the same one as in the Academy Awards, and they set the standard ratio back in 1932.note It was the uprising of television in the fifties that led to the film industry seeking out methods of innovation to stay competitive. A lot of attempts at innovation were gimmicky and didn't last long—the first round of 3D, for instance—but two, the increased use of Color film and wider aspect ratios, proved enduring. Various ratios were tried; the widest ratio of all was Cinerama, which used three projectors to display a 2.89:1 image.note One should keep in mind that wider aspect ratio is not necessarily larger; proper IMAX is the largest screen in the world, but only has a ratio of 1.43:1.
Another aspect ratio that's becomming more common is 18:9, or 2:1, also known as Univisium. This aspect ratio was first seen all the way in the 1950s, although it wasn't until a proposal in 1998 that this could be adopted as a good compromise between the other ratios in usenote that it started to be more common. Arguably it took another two decades until the mid-2010s though for 18:9 to really start becoming a thing, as both content producers and smartphone manufacturers both took to it (although for different reasons - TV started using 18:9 as it made content feel more cinematic while avoiding too much letterboxing as would occur with a jump to 2.39:1, while smartphones started using 18:9 as companies realized it would make the phone bigger without making it too large to hold).
Because the Widescreen Shot of a feature film is so often different from that of a television screen, a movie image will not fit squarely within a TV screen when released for home video or shown on broadcast television. In order to get around this problem, the aspect ratio of the film must be altered. There are a few common methods for doing this: Pan and Scan, Letterbox, and Visual Compression.
As people still hold onto earlier media as Technology Marches On, some have noticed that older DVDs that advertise themselves as Letterbox are actually designed for 4:3 screens and will not "fill" newer TV sets, despite matching the aspect ratio on a smaller scale. This is because they were released when DVD was taking off in the late 90's when almost every computer monitor and TV screen was 4:3. Some newer televisions have a zoom function for this, though the zoomed image may seem pixelated.
Multiple aspect ratios have been occasionally used by directors and game designers and like for cinematic reasons. For example, in The Grand Budapest Hotel 4:3 is used for some scenes to convey it's the olden days (since 4:3 has started to fall into disuse in favor of 16:9), and video games might have cutscenes in 21:9 to give the appearance of being a movie (21:9 being the closest aspect ratio to 2.39:1). There are a couple of reasons to do this; first, changing the aspect ratio alerts the player that they are no longer in control of what is happening and can just watch the story; second, if the cutscene is pre-rendered, the black bars compress down to almost zero (or can be omitted from the encode and added in again at playback), letting you drop the bitrate on the compression slightly, shaving a small amount off the overall file size - which, if you have many hours of cutscenes, can add up to a significant space saving.
Aspect Ratio can also be an issue with cameraphone footage. Charlie Brooker has complained about people filming with their phones held upright, producing a tall, narrow image instead of the "correct" widescreen ratio.note Glove and Boots would like you to take three minutes to watch their PSA on the subject here. If you want to learn more about the evolution of the Aspect Ratio in movie history, check out this webcast.