Aspect ratio

Aspect ratio is the relationship between the width of an object and its height. A modern desktop digital display, for example, typically has an aspect ratio of 16:9. Older CRT displays usually had a ratio of 4:3.

When expressing an aspect ratio, the width is always the first number, and the height is always the second. This immediately tells you whether an element is in landscape or portrait format: if the first number is larger, it’s landscape; if the second number is larger, it’s portrait.