When you buy a device, you will often see both screen size and resolution listed in the specs. The screensize is the physical measurement diagonally of the screen in inches. This is not to be confused with the resolution, which is the number of pixels on the screen often displayed as a width by height (i.e. 1024×768). Because devices with the same screen size can have very different resolutions, developers use viewports when they’re creating mobile friendly pages. Viewports are scaled down versions of resolutions that allows sites to be viewed more consistently across different devices. Viewports are often more standardized and smaller than resolution sizes.

While desktop and laptop displays are in landscape orientation (wider than tall), many mobile devices can be rotated to show websites in both landscape and portrait (taller than wide) orientations. This means that designers and developers must design for these differences.