The aspect ratio of an image describes the proportional relationship between its width and its height. It is commonly expressed as two numbers separated by a colon, as in "16:9". For an "x":"y" aspect ratio, no matter how big or small the image is, if the width is divided into "x" units of equal length and the height is measured using this same length unit, the height will be measured to be "y" units. 

In, for example, a group of images that all have an aspect ratio of 16:9, one image might be 16 inches wide and 9 inches high, another 16 centimeters wide and 9 centimeters high, and a third might be 8 yards wide and 4.5 yards high. 

The most common aspect ratios used today in the presentation of films in cinemas are 1.85:1 and 2.39:1. Two common videographic aspect ratios are 4:3 (1.3:1), the universal video format of the 20th century, and (1.7:1), universal for high-definition television and European digital television. Other cinema and video aspect ratios exist, but are used infrequently. 

In still camera photography, the most common aspect ratios are 4:3, 3:2, and more recently being found in consumer cameras 16:9. Other aspect ratios, such as 5:3, 5:4, and 1:1 (square format), are used in photography as well, particularly in medium format and large format.
any others used?
In still camera photography, the most common aspect ratios are 4:3, 3:2