In compositing, the proportions of an image expressed as the ratio between the horizontal and vertical dimensions.

Because pixels are not necessarily proportional, the aspect ratio is independent of the number of pixels in the X and Y directions. For example, both NTSC and PAL television screens are 4 x 3 (for an aspect ratio of 1.33). However, a CCIR601 NTSC image is 720 x 486 pixels, while a PAL image is 720 x 576 pixels.