Common Aspect Ratios: 16:9, 4:3, 3:2, 1:1, 4:5, 21:9 and What They Are For

An aspect ratio is the shape of an image: width compared to height, written with a colon. A 16:9 video frame is 1.78 times wider than it is tall; a 4:5 portrait photo is narrower than it is tall (0.8)