In telecommunications, keying is a form of modulation where the modulating signal takes one of two values at all times. For example: "on" or "off", "mark" or "space". The name derives from the Morse code key used for telegraph signalling.

In graphics[?], keying is an informal term for compositing two full frame images together, by discriminating the visual information into values of color and light.

• A chroma key is the removal of a color from one image to reveal another "behind" it. The best known example is television weather broadcasts, where the meteorologist is filmed in front of a flat, even colored green (or blue) screen. The background color is removed electronically, and replaced with a computer graphic map which the meterologist points to (by glancing at monitors hidden off-camera). The background color, of course, must not also appear as part of the meteorologist's clothing, else one might see, for example, the heart of Texas deep in the heart of the forecaster! Chroma keys are done most commonly in front of a bluescreen or greenscreen, because those colors are easiest to avoid in clothing and flesh tones.

• A luma key similarly replaces color from an images which falls into a particular range of brightness. This technique is less controllable, but can be used on graphic elements.

• A matte key uses three images: the two images that will be composited, and a black-and-white third image that dictates the blending of the two, with white revealing one image, black the other, and grey revealing a blend of the two together.

Generally, the "bottom" image is called the beauty, the image that appears on top is the fill and the discriminating element (chroma, luma or matte) is called the key or matte.