Compose key

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The compose key and compose LED on a 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

On some computer systems, a compose key is a key which is designated to signal the software to interpret the next keystrokes as a combination in order to produce a character not found on the keyboard.

For example, pressing compose, then a, then e may produce the AE ligature, æ. Pressing compose, then ', then e can yield an e with an acute accent, é.

The compose key is known as "Multi_key" in the X Window System. 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 (most often the "Context Menu" key, since "Start" is already used to open the start menu). It can also be specified in XkbOptions (for example, "compose:rwin"). Multi_key can also be assigned with the xmodmap(1) utility.

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[1]. However, this symbol doesn't appear to be used by the industry.

Some common compose combinations
' a á
' A Á
" a ä
" A Ä
` a à
` A À
~ a ã
~ A Ã
^ a â
^ A Â
o a å
o A Å
Vowels support most of the above
s s ß
, c ç
, C Ç
O R ®
O C ©
< < «
> >  »
" <
" >
' <
' >
. . ·
x x ×
-  : ÷
^ 0 °
^ 1 ¹
^ 2 ²
^ 3 ³
s o/0 §
1 2 ½
1 4 ¼
3 4 ¾
/ O ø
/ O Ø
- d ð
- D Ð
~ n ñ
t h þ
a e æ
 !  ! ¡
 ?  ? ¿
- L £
= E
= Y ¥
| c ¢
o x ¤
/ / \

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 keyboard since the 1990s. In particular the $ compose sequence ceased to work on modern systems. The historic backslash sequence (compose-//) witnesses of the similar situation for this key and is still working on modern systems.

The table shown 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 customisable compose sequences and Unicode, the table would be far from complete. Given the vast number of sequences permissible and the vast number of characters desirable in Unicode, a complete table would be incredibly long.

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.

On Mac OS X, the Option key provides access to some extended characters. For example, holding down option and pressing a results in å; holding down option and pressing ` will create a highlighted ` character which will be added to the next letter if possible - so if an e is then pressed, the resultant character is è, while if an r is pressed the two characters are not compatible so the result is `r.

Microsoft Windows does not use a compose key, but it does support dead keys and AltGr for many characters. Other characters can be generated by holding down Alt, and then typing the ASCII code or Unicode code corresponding to the desired character on the numeric keypad. A few programs such as putty also directly provide Compose key support.

There are also a number of open source utilities and installable keyboard layouts available that emulate the compose key using a selectable modifier key like Control or AltGr.

In AmigaOS the compose key is again not present on the keyboard layout, even if the system supports option keys named "Left Amiga" and "Right Amiga" similar to option key of Macintosh but with slight different usage. In Amiga dead keys are called "deaf keys" and are generated by the pressing of ALT key (Eg: "ALT-F" combination of keys + "a" key = á; "ALT-F" + "e" = é; "ALT-G" + "a" = à; "ALT-G" + "e" = è; etc.). AmigaOS was also the first Operating System to avoid using its own standards, and pragmatically Commodore adopted officially for Amiga an international approved standard ANSI ISO8859-1 layout for its internal codepage operations and thus also for its keyboard layout.

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