Compose key
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 keyboard LED or other physical indicator that a compose sequence is ongoing, though the OS may show some icon on the user's text input field.
Compose sequences
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. This makes it possible for experienced typists to enter composed characters rapidly.Earlier versions of compose sequences followed handwriting and the overstrike technique by putting the letter first and diacritics second. For example produced the character ñ. This order is still in use, however the inverse order known from accent-mark dead keys present on the last typewriters is used today: for ñ. This allows multiple diacritics, for instance typing for ấ.
Non-accented characters are generally constructed from letters that when overtyped or sequenced would produce something like the character. For instance will produce the copyright symbol ©, and will produce Æ.
There is no intrinsic limit on sequence length, which should respect both the rules of mnemonics and ergonomics, and feasibility within a comprehensive compose tree. For example, might be inserted by, where indicates circled characters, indicates inverse, indicates sans-serif, and indicates the final character.
Compared to other input methods
The primary advantage of a compose key is that the sequence used to select the character can be made up of any letters, numbers, or 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.The primary disadvantage is that compose sequences always require at least one more keystroke. Inconvenient placement of the compose key can also slow typing.
The most common method used to enter characters not printed on the keys is a modifier key such as AltGr. This method suffers from the limitation that AltGr adds just two more associations for each key. The additional characters made available are typically customized to the local market and thus meet the needs of most users, most of the time. The default UK/Ireland layout, for example, provides the letters á é í ó ú that are needed for Irish Gaelic but consequently not the letters à è ì ò ù needed for Scottish Gaelic.
A dead key treats the first character in any sequence as a modifier for the next key: when pressed, the key appears to have no effect until the next key is pressed.. For example, the grave key is often a dead key: to achieve, the sequence is used. Compound sequences are possible: for example to add a two dots, the seems an obvious candidate but clearly this heavily used key cannot be repurposed as a dead key. The solution is to modify it using AltGr, thus delivers. Other combinations are rather less obvious than their compose-key counterparts: for example compare with for .
Alt codes or Unicode numerical input could almost be considered a compose key, but use 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.