Aspect ratio
The aspect ratio of an image (or any rectangular thing) is ratio of its height and width.
Many different aspect ratios have been used, and continue to be used, in photography. Some have recurred several times. In film (and earlier, plate) photography, the ratios 3:2 and 4:3 occur in several sizes of media. The 4:3 ratio arises naturally as the half-frame adaptation of a film used with a 3:2 full-size frame. Similarly, some plate sizes follow natural patterns, since some small plates were made by cutting a larger size in half.
The 120 film was designed to give eight pictures with a 3:2 aspect ratio (2¼x3¼ inch), but was soon used in cameras adapted for sixteen pictures with a 4:3 ratio (2¼x1⅝ inch), and twelve 1:1 (i.e. square) pictures. The 6x7 format did not follow until much later.
The 3:2 ratio became the most common used with 35mm film. Early cameras for 35mm film used a variety of other frames, however. Again, there are many 4:3 half-frame 35mm cameras. There are rather few square-format 35mm cameras.
The once-popular 126 film format was devised for 1:1 square format.
Almost all digital camera models today natively use the 4:3 ratio. There are few exceptions to this rule among compact cameras but the majority of dSLR cameras use 3:2 sensors. A small number of compact cameras have had 3:2, 16:9 and 5:4 aspect ratio sensors.
An assortment of aspect ratios are used with photographic prints. Photographic paper sizes offered commercially retained traditional plate sizes (e.g. 5x7 and 8x10 inch) for years after 35mm became the most popular format among amateurs. Images were often cropped to fit.
