Compose key

The compose key and compose LED on Sun Type 5, 6 and 7 keyboards is the second-last key on the bottom row.

The compose key on a DEC LK201 keyboard is the leftmost key on the bottom row.

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.

Usage

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.

In a loose sense, the character produced with a compose-key sequence can be seen as a precomposed character, but it does not need to be technically decomposable or have any visual resemblance with the characters normally produced with the same keys (i.e., without striking the compose key first). Nevertheless, there is usually some kind of an association that helps to make the key sequence more mnemonic. For example, typing Compose followed by T and then H may produce the Icelandic (as well as Old Norse and Old English) letter þ (thorn), which phonetically corresponds to the digraph th in, e.g., thing.

Compose key in various computer systems

The compose key is known as "Multi_key" in the X Window System, and must be interpreted by the client program (typically Xlib), not the server. In XFree86 and X.Org Server, many keyboard layouts have a variant that maps Multi_key to some key, usually (on PC keyboards) to either of the Windows keys, or ⇧ Shift+AltGr ^[1] or sometimes ⇧ Shift+Right-Ctrl.^{[citation needed]} It can also be specified in XkbOptions (for example, "compose:rwin"). Multi_key can also be assigned with the xmodmap(1) utility. Note that it is common for ⇧ Shift+AltGr to be mapped to the compose key, but for AltGr+⇧ Shift to be mapped to the fourth "keyboard level" modifier.^[2] The first three levels correspond to holding no modifiers, ⇧ Shift, and AltGr respectively. So for example the copyright symbol, © can be typed using the compose key as ⇧ Shift+AltGr, o, c, or using the level four modifier as AltGr+⇧ Shift+c.

ISO/IEC 9995-7 assigned to this key a graphical symbol ⎄, which exists in Unicode as U+2384 since version 3.0.0 of the standard.^[3] However, this symbol doesn't appear to be used by the industry.

Originally the currency sign on the keyboards were nationalized, and not every keyboard sold in the world contained a dollar sign ($), which explains why it appears there^{[clarification needed]}, although it is part of any standard^{[by whom?]} keyboard since the 1990s. In particular the $ compose sequence ceased to work^[when?] on modern^[which?] systems. The historic backslash sequence (compose-//) witnesses of the similar situation for this key and is still working on modern systems.

Microsoft Windows does not use a compose key as such, but a few programs such as PuTTY provide compose key support. There are also a number of open source utilities (such as AllChars) and installable keyboard layouts available that emulate the compose key using a selectable modifier key like Control or AltGr.

Common compose combinations

The table shown below contains the current common compositions for Xorg 7. Other compositions may work, based on the de-facto Sun/DEC/Falco standard. Particularly for modern systems which support customizable compose sequences and Unicode, the table would be far from complete. Given the vast number of sequences permissible and the vast number of characters available in Unicode, a complete list would be very long.

Hitting compose then this and then this renders this ' a á ' A Á " a ä " A Ä ` a à ` A À ~ a ã ~ A Ã ^ a â ^ A Â c a ǎ c A Ǎ o a å o A Å _ a ā _ A Ā  ; a ą  ; A Ą b a ă b A Ă Vowels support most of the above

Hitting compose then this and then this renders this s s ß , c ç , C Ç O R ® O C © t m ™ < < « > > » " < “ " > ” ' < ‘ ' > ’ . . … . ^ · x x × -  : ÷ ^ 0 ⁰ ^ 1 ¹ ^ 2 ² ^ 3 ³ 1 2 ½ 1 4 ¼ 3 4 ¾

Hitting compose then this and then this renders this s o § / o ø / O Ø - d đ - D Đ d h ð D H Ð ~ n ñ t h þ T H Þ a e æ A E Æ  !  ! ¡  ?  ? ¿ - L £ = E € = Y ¥ | c ¢ o x ¤ / / \ o o °

See also

• Modifier key
• Dead key
• AltGr key
• Windows Alt keycodes
• Precomposed character
• Combining character

References

1. ^ https://help.ubuntu.com/community/ComposeKey
2. ^ [1]
3. ^ Unicode chart 2300-23FF

External links

• Linux Compose Key Sequences with equivalent unicode mappings
• XFree86 Compose Key character map by XFree86
• Everything2's list of Compose Key combinations
• SUN's Compose Key combinations used under CDE
• Setting up Compose Key in the X Window System
• AllChars, an open-source utility that emulates the Compose key under MS-Windows
• JLG Extended Keyboard Layout - yet another utility based on the Compose Key mechanism under Windows
• Installable keyboard layouts for Windows with source for editing in the MSKLC keyboard editor
• Accent Composer - Compose accented characters and other symbols under Microsoft Windows. Unicode compatible
• Compose key sequences used in GNOME/GTK