A compose key is a key on a computer keyboard that indicates that the following 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.
History
The Compose Character key was introduced by engineers at Digital Equipment Corporation 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. 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 LED or other indicator that a compose sequence is ongoing.
The primary advantage of a compose key is that the entire sequence used to select the character is made up of any letters and numbers and symbols available on the keyboard. This allows the sequence to be more mnemonic, so it is easier to remember, possible to guess at if unknown, and can support far greater numbers of characters. For instance if a dead key system where all the dead keys are marked with diacritics might require for ß, the compose key requires typing, matching the common substitution with "SS" for the character. Another example is ⅔ which is entered by, whereas a single dead key solution turns out to be something like . Modifier keys are even more limited as typically only one key from the keyboard is used to select the character. Alt codes or Unicode numerical input could almost be considered a compose key, but with unintuitive numbers, instead of mnemonics, as the selector. Modern GUI character choosers often require a search function that is not much different than the compose sequences to locate a character quickly. The primary disadvantage is that compose sequences always require at least one more keystroke. Inconvenient placement of the compose key can also slow typing.
X header files call the Compose Key the "Multi_key". On Xorg the default Compose Key is +,. As this is rather inconvenient it is common to select a keyboard layout where another key such as the right-hand or is mapped to the compose key, this option is normally available in the settings of the desktop environment. The X keyboard driver does not allow the key used for Compose to also function as a modifier. On modern systems a vast number of compose sequences are supported.
Windows
On Microsoft Windows a few programs such as PuTTY provide compose-key support. To emulate the compose key for all software, keyboard shortcututilities are often involved. There are also a number of open-source utilities. Installable keyboard layouts are available that contain a compose key assigned to one of the keys like or. They work by using the dead-key chaining feature that is more commonly used to input letters with multiple diacritics. Such keyboard layouts can also be programmed directly in C, compiled using the free-of-charge Windows Driver Kit, and packaged using the free-of-charge Microsoft Keyboard Layout Creator 1.4, compatible up to the latest versions of the OS.
macOS
Although the Cocoa text input system allows entry of many alternate and accented characters natively in macOS, a true compose-key solution isn't built in. At least one has been implemented using the Karabiner utility. That works with all applications, as does the use of keyboard drivers where Compose is implemented using the dead key chaining feature.
Although Chrome OS is supplied with a larger repertoire of glyphs than most competitors, the chords needed to achieve them are not always as obvious as the Compose concept provides. Google has made available an add-on to compete in this market.
The table shown below shows some of the default compositions for the X.Org server. For modern systems which support Unicode, the table below is far from complete.
