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Concept
Compose key
Summary
A compose key (sometimes called multi key) is a key on a computer keyboard that indicates that the following (usually 2 or more) keystrokes trigger the insertion of an alternate character, typically a precomposed character or a symbol.
For instance, typing followed by and then will insert ñ.
Compose keys are most popular on Linux and other systems using the X Window System, but software exists to implement them on Windows and macOS.
The Compose Character key was introduced by engineers at Digital Equipment Corporation (DEC) on the LK201 keyboard, available since 1983 with the VT220 terminal. The keyboard included an LED indicating that a Compose sequence is on-going. While the LK201 introduced the group of command keys between the alphanumerical block and the numerical keypad, and the "inverted T" arrangement of arrow keys, which have become standard, the compose key by contrast did not become a standard.
In 1987, Sun Microsystems released the Sun4, the first dedicated Unix workstation that had a compose key. On the keyboards of Sun Type 5 and 6 workstations, the Compose LED is placed in the keycap (see picture below).
ISO/IEC 9995-7 designed a graphical symbol for this key, in ISO/IEC 9995-7 as symbol 15 "Compose Character", and in ISO 7000 "Graphical symbols for use on equipment" as symbol ISO-7000-2021. This symbol is encoded in Unicode as .
Because Microsoft Windows and macOS do not support a compose key by default, the key does not exist on most keyboards designed for modern PC hardware. When software supports compose key behaviour, some other key is used. Common examples are the right-hand Windows key, the key, or one of the keys. There is no keyboard LED or other physical indicator that a compose sequence is ongoing, though the OS may show some icon (such as the "Compose Character" symbol, below) on the user's text input field.
If the Compose key is not also a modifier key, then key rollover means the compose key does not have to be released before the subsequent keystrokes.
Official source
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