A compose key, available on some computer keyboards, is a special kind of modifier key designated to signal the software to interpret the following sequence of two (or more) keystrokes as a combination in order to produce a character not found directly on the keyboard. For example, striking Compose followed by A and then E may produce the ligature æ, whereas striking Compose followed by O and then C can yield © (circled C, the copyright symbol).

The compose key can be found on the LK201 family of keyboards from Digital Equipment Corporation and its successors. The key can also be found on keyboards from Sun Microsystems. However, any key on a keyboard can be configured to serve as a compose key, as long as the functionality is supported by the software in use.

The compose key is different from a typical modifier key (such as AltGr or ⌥ Option) in that, rather than being pressed and held while another key is struck, it is pressed and released before striking the keys to be modified. Typically, the text-entry system does not indicate that any keys have been pressed until the composed character appears after the final keystroke of a predefined sequence. Therefore, the compose key can also be described as a kind of generic dead key. What makes the compose key different from a dead key is that while each dead key can only be used to attach a specific diacritic to various base letters, the compose key can be used to produce any kind of a character supported by the character set and font in use. This way, an even wider variety of special characters can be made available on a keyboard with a limited number of keys.

Chroma key compositing, or chroma keying, is a special effects / post-production technique for compositing (layering) two images or video streams together, used heavily in many fields to remove a background from the subject of a photo or video - particularly the newscasting, motion picture and videogame industries. A color range in the top layer is made transparent, revealing another image behind. The chroma keying technique is commonly used in video production and post-production. This technique is also referred to as color keying, colour-separation overlay (CSO; primarily by the BBC), or by various terms for specific color-related variants such as green screen, and blue screen - chroma keying can be done with backgrounds of any color that are uniform and distinct, but green and blue backgrounds are more commonly used because they differ most distinctly in hue from most human skin colors and no part of the subject being filmed or photographed may duplicate a color used in the background. It is commonly used for weather forecast broadcasts, wherein the news presenter appears to be standing in front of a large map during live television newscasts, though in actuality it is a large blue or green background. When using a blue screen, different weather maps are added on the parts of the image where the color is blue. If the news presenter wears blue clothes, his clothes will also be replaced with the background video. A complementary system is used for green screens. Chroma keying is also used in the entertainment industry for special effects in movies and videogames. The advanced state of the technology and much commercially available computer software, such as Pinnacle Studio, Adobe Premiere, and dozens of other computer programs, makes it possible and relatively easy for the average home computer user to create videos using the "chromakey" function and easily affordable greenscreen or bluescreen kits.