Type a width or a height and the other dimension follows from the ratio you have chosen, so images and video stay proportional. Useful for resizing exports, sizing embeds, and checking that a crop will not letterbox.
An aspect ratio is width divided by height. Any two dimensions with the same quotient look identical, just larger or smaller. Keeping the ratio fixed is what stops an image stretching, and it is why 1920x1080 and 1280x720 are interchangeable for framing.
16:9 is the default for video and displays. 9:16 is its vertical twin for phone-first platforms. 4:5 is the tallest ratio most social feeds will show without cropping, which makes it efficient for reaching people scrolling. 2.39:1 is the cinema standard and is what puts black bars on a 16:9 television.
16:9 is the standard for landscape video on YouTube and television. 9:16 is vertical for Shorts, Reels and TikTok. 1:1 square and 4:5 portrait perform well in social feeds.
It is the marketing name for ultrawide displays, which are usually 64:27 (about 2.37:1) rather than exactly 21:9. Cinema widescreen is 2.39:1, which is close enough that films fill an ultrawide screen with only a thin border.
Steuervergleich Kanton Zürich — Gemeindesteuern vergleichen (Deutsch)