At the start most films and analog television formats around the world had a ratio of 4:3note (1.33:1, very close to cinema's "Academy Ratio" of 1.375:1), 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") or 2.39:1 (also known as CinemaScope or "Scope"). Most silent films used the 4:3 aspect ratio, whereas "talkies" used "Academy" ratio of 1.375:1. The latter is the one the Academy Awards set as the standard ratio back in 1932.note It was the uprising of television in The '50s 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
Another aspect ratio that's becoming 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. Some phones even go a step further with 19:9 displays).
The logistics of what aspect ratio a production would use depends on the genre and tone they are looking for. 4:3 became known for its use in comedy, drama, or other small scale projects while 16:9 became known for action, science fiction, "epic scope", and other large scale projects. The reasons were that the more square frame pushes for tighter composition of the characters and vertical alignment (showing large structures and moving up or down), while a more rectangular frame widens the field of view and horizontal movement (more space for large crowds and a Chase Scene). Note that a wider aspect ratio is not necessarily more picture. Both formats can be, and have been, cropped during the editing process to resemble the other, and in experiments with aspect ratios in the 50's some movies were intended to be done so in different showings. The IMAX format is a stock selling point in modern movies for the action sequences, not only for the 1.43:1 aspect ratio but also filmed on 70mm film (the standard film stock is 35mm, which results in a spike in clarity and resolution). To facilitate this the IMAX Grand Theatres (GT), known by movie buffs as "true" IMAX theaters, have the largest screens in the world.
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 (hereby P&S), Letterbox, Open Matte, and Visual Compression.
VHS and Betamax releases typically used open matte when possiblenote , and P&S when it's notnote . Some letterboxed VHS tapes exist, but are uncommon. LaserDisc started releasing letterbox versions of almost every movienote (alongside some P&S versions) in The '90s, as it was seen as the format of film buffs and A/V geeks.
While P&S and open matte DVDs exist, they are uncommon. DVD was credited with killing P&S and popularizing letterbox, to the delight of film buffs and directors. DVD had two aspect ratio modes: 4:3 and anamorphic widescreen. Both used 720x480 resolution (or 720x576 for PAL/SECAM materials)note . 4:3 mode was used for 4:3 and Academy Ratio content. Anamorphic mode squeezes a 16:9 image along the horizontal axis and sets a flag bit in the header data, so that if your DVD player and 16:9 TV were correctly set up the image would be restored to the correct width when played back. (If they were not correctly set up, everyone would look alarmingly thin; the reverse was true for 4:3 materials stretched incorrectly, as if they were 16:9, which made everyone look fat - some absolute monsters were apparently completely okay with this because it didn't have those annoying black bars on the sides, and never mind that it turned everyone into later-career Marlon Brando). On a 4:3 display, the player would generate the letterbox itself. This allowed widescreen images to use more of the available resolution, compared to a 16:9 image letterboxed into 4:3. For native 16:9 content, the letterbox is completely gone.
Unfortunately, some older DVDs that advertise themselves as Letterbox actually use 4:3 mode, imitating older letterboxed VHS and LaserDisc titles. These will not "fill" newer widescreen TV sets, despite matching (or wider) the aspect ratio on a smaller scale. This is because they were released when DVD was taking off in the late 90's and almost no one had a widescreen computer monitor or TV screen. Some newer widescreen televisions have a zoom function for this, though the zoomed image may seem pixelated or blurry.
Multiple aspect ratios have been occasionally used by directors and game designers and like for cinematic reasons. For example, in The Grand Budapest Hotel Academy Ratio is used for some scenes to convey it's the olden days (since in cinema it has started to fall into disuse in favor of widescreen in The '50s), and video games might have cutscenes in 21:9note 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. Meanwhile, Brainstorm switched between two ratios; the "real world" sequences were filmed in spherical 35mm and have a 1.66:1 aspect ratio. The "Brainstorm" sequences were shot with Super Panavision 70 and are in 2.20:1 aspect ratio.
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.