Compose Key

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

The compose key is different from a typical modifier key (such as AltGr) 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.

Though it can be considered a type of dead key, the compose key differs in that a normal dead key produces part of the composite, typically a diacritic, to combine with a following letter. Since each different first part requires a different dead key, this limits the number combinations. The first part of the composite from a compose key can be any key on the keyboard, vastly increasing the number of possible combinations, and allowing some easily memorized sequences such as 1,2 for ½. However it has the cost that using one more keystroke than a dead key.