<s>
A	O
compose	B-Device
key	I-Device
(	O
sometimes	O
called	O
multi	B-Device
key	I-Device
)	O
is	O
a	O
key	O
on	O
a	O
computer	B-Device
keyboard	I-Device
that	O
indicates	O
that	O
the	O
following	O
(	O
usually	O
2	O
or	O
more	O
)	O
keystrokes	O
trigger	O
the	O
insertion	O
of	O
an	O
alternate	O
character	O
,	O
typically	O
a	O
precomposed	O
character	O
or	O
a	O
symbol	O
.	O
</s>
<s>
Compose	B-Device
keys	I-Device
are	O
most	O
popular	O
on	O
Linux	B-Application
and	O
other	O
systems	O
using	O
the	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
,	O
but	O
software	O
exists	O
to	O
implement	O
them	O
on	O
Windows	B-Application
and	O
macOS	B-Application
.	O
</s>
<s>
The	O
Compose	O
Character	O
key	O
was	O
introduced	O
by	O
engineers	O
at	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
on	O
the	O
LK201	B-Device
keyboard	B-Device
,	O
available	O
since	O
1983	O
with	O
the	O
VT220	B-Device
terminal	O
.	O
</s>
<s>
The	O
keyboard	B-Device
included	O
an	O
LED	O
indicating	O
that	O
a	O
Compose	B-Device
sequence	I-Device
is	O
on-going	O
.	O
</s>
<s>
While	O
the	O
LK201	B-Device
introduced	O
the	O
group	O
of	O
command	B-Device
keys	I-Device
between	O
the	O
alphanumerical	O
block	O
and	O
the	O
numerical	O
keypad	O
,	O
and	O
the	O
"	O
inverted	O
T	O
"	O
arrangement	O
of	O
arrow	O
keys	O
,	O
which	O
have	O
become	O
standard	O
,	O
the	O
compose	B-Device
key	I-Device
by	O
contrast	O
did	O
not	O
become	O
a	O
standard	O
.	O
</s>
<s>
In	O
1987	O
,	O
Sun	O
Microsystems	O
released	O
the	O
Sun4	B-Device
,	O
the	O
first	O
dedicated	O
Unix	O
workstation	O
that	O
had	O
a	O
compose	B-Device
key	I-Device
.	O
</s>
<s>
On	O
the	O
keyboards	B-Device
of	O
Sun	O
Type	O
5	O
and	O
6	O
workstations	O
,	O
the	O
Compose	O
LED	O
is	O
placed	O
in	O
the	O
keycap	O
(	O
see	O
picture	O
below	O
)	O
.	O
</s>
<s>
Because	O
Microsoft	B-Application
Windows	I-Application
and	O
macOS	B-Application
do	O
not	O
support	O
a	O
compose	B-Device
key	I-Device
by	O
default	O
,	O
the	O
key	O
does	O
not	O
exist	O
on	O
most	O
keyboards	B-Device
designed	O
for	O
modern	O
PC	O
hardware	O
.	O
</s>
<s>
When	O
software	O
supports	O
compose	B-Device
key	I-Device
behaviour	O
,	O
some	O
other	O
key	O
is	O
used	O
.	O
</s>
<s>
Common	O
examples	O
are	O
the	O
right-hand	O
Windows	B-Device
key	I-Device
,	O
the	O
key	O
,	O
or	O
one	O
of	O
the	O
keys	O
.	O
</s>
<s>
There	O
is	O
no	O
LED	O
or	O
other	O
indicator	O
that	O
a	O
compose	B-Device
sequence	I-Device
is	O
ongoing	O
.	O
</s>
<s>
If	O
the	O
Compose	B-Device
key	I-Device
is	O
not	O
also	O
a	O
modifier	B-Device
key	I-Device
,	O
then	O
key	B-Device
rollover	I-Device
means	O
the	O
compose	B-Device
key	I-Device
does	O
not	O
have	O
to	O
be	O
released	O
before	O
the	O
subsequent	O
keystrokes	O
.	O
</s>
<s>
Earlier	O
versions	O
of	O
compose	B-Device
sequences	I-Device
followed	O
handwriting	O
and	O
the	O
overstrike	B-Application
technique	O
by	O
putting	O
the	O
letter	O
first	O
and	O
diacritics	O
second	O
.	O
</s>
<s>
This	O
order	O
is	O
still	O
in	O
use	O
,	O
however	O
the	O
inverse	O
order	O
known	O
from	O
accent-mark	O
dead	B-Device
keys	I-Device
present	O
on	O
the	O
last	O
typewriters	O
is	O
used	O
today	O
:	O
for	O
ñ	O
.	O
</s>
<s>
The	O
primary	O
advantage	O
of	O
a	O
compose	B-Device
key	I-Device
is	O
that	O
the	O
sequence	O
used	O
to	O
select	O
the	O
character	O
can	O
be	O
made	O
up	O
of	O
any	O
letters	O
,	O
numbers	O
,	O
or	O
symbols	O
available	O
on	O
the	O
keyboard	B-Device
.	O
</s>
<s>
The	O
primary	O
disadvantage	O
is	O
that	O
compose	B-Device
sequences	I-Device
always	O
require	O
at	O
least	O
one	O
more	O
keystroke	O
.	O
</s>
<s>
Inconvenient	O
placement	O
of	O
the	O
compose	B-Device
key	I-Device
can	O
also	O
slow	O
typing	O
.	O
</s>
<s>
The	O
most	O
common	O
method	O
used	O
to	O
enter	O
characters	O
not	O
printed	O
on	O
the	O
keys	O
is	O
a	O
modifier	B-Device
key	I-Device
such	O
as	O
AltGr	B-Device
.	O
</s>
<s>
This	O
method	O
suffers	O
from	O
the	O
limitation	O
that	O
AltGr	B-Device
adds	O
just	O
two	O
more	O
(	O
shifted	O
and	O
unshifted	O
)	O
associations	O
for	O
each	O
key	O
.	O
</s>
<s>
The	O
default	O
UK/Ireland	O
layout	O
,	O
for	O
example	O
,	O
provides	O
(	O
via	O
AltGr+aeiou	O
)	O
the	O
letters	O
á	O
é	O
í	O
ó	O
ú	O
that	O
are	O
needed	O
for	O
Irish	O
Gaelic	O
but	O
consequently	O
not	O
the	O
letters	O
à	O
è	O
ì	O
ò	O
ù	O
needed	O
for	O
Scottish	O
Gaelic	O
.	O
</s>
<s>
A	O
dead	B-Device
key	I-Device
treats	O
the	O
first	O
character	O
in	O
any	O
sequence	O
as	O
a	O
modifier	O
for	O
the	O
next	O
key[s]	O
:	O
when	O
pressed	O
,	O
the	O
key	O
appears	O
to	O
have	O
no	O
effect	O
until	O
the	O
next	O
key	O
is	O
pressed	O
.	O
</s>
<s>
(	O
Most	O
systems	O
try	O
to	O
make	O
the	O
choice	O
of	O
dead	B-Device
key	I-Device
a	O
logical	O
part	O
of	O
the	O
character	O
selection	O
;	O
this	O
makes	O
sense	O
for	O
accent	O
marks	O
and	O
the	O
concept	O
is	O
extended	O
to	O
other	O
symbols	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
grave	O
(	O
"	O
backtick	O
"	O
)	O
key	O
(	O
)	O
is	O
often	O
a	O
dead	B-Device
key	I-Device
:	O
to	O
achieve	O
,	O
the	O
sequence	O
is	O
used	O
.	O
</s>
<s>
Compound	O
sequences	O
are	O
possible	O
:	O
for	O
example	O
to	O
add	O
a	O
two	O
dots	O
(	O
diacritic	O
)	O
,	O
the	O
seems	O
an	O
obvious	O
candidate	O
but	O
clearly	O
this	O
heavily	O
used	O
key	O
cannot	O
be	O
repurposed	O
as	O
a	O
dead	B-Device
key	I-Device
.	O
</s>
<s>
The	O
solution	O
is	O
to	O
modify	O
it	O
using	O
AltGr	B-Device
,	O
thus	O
delivers	O
.	O
</s>
<s>
Other	O
combinations	O
are	O
rather	O
less	O
obvious	O
than	O
their	O
compose-key	O
counterparts	O
:	O
for	O
example	O
compare	O
with	O
for	O
(	O
ChromeOS	B-Operating_System
with	O
UK	O
extended	O
mapping	O
)	O
.	O
</s>
<s>
Alt	O
codes	O
or	O
Unicode	O
numerical	O
input	O
could	O
almost	O
be	O
considered	O
a	O
compose	B-Device
key	I-Device
,	O
but	O
use	O
unintuitive	O
numbers	O
,	O
instead	O
of	O
mnemonics	O
,	O
as	O
the	O
selector	O
.	O
</s>
<s>
Modern	O
GUI	O
character	O
choosers	O
often	O
require	O
a	O
search	O
function	O
that	O
is	O
not	O
much	O
different	O
than	O
the	O
compose	B-Device
sequences	I-Device
to	O
locate	O
a	O
character	O
quickly	O
.	O
</s>
<s>
X	B-Operating_System
header	O
files	O
call	O
the	O
Compose	B-Device
Key	I-Device
the	O
"	O
Multi_key	O
"	O
.	O
</s>
<s>
On	O
Xorg	B-Operating_System
the	O
default	O
Compose	B-Device
Key	I-Device
is	O
+	O
,	O
(	O
while	O
pressing	O
before	O
is	O
the	O
"	O
fourth	O
keyboard	B-Device
level	O
modifier	O
"	O
,	O
a	O
different	O
key	O
)	O
.	O
</s>
<s>
As	O
this	O
is	O
rather	O
inconvenient	O
(	O
especially	O
for	O
keyboards	B-Device
without	O
an	O
)	O
it	O
is	O
common	O
to	O
select	O
a	O
keyboard	B-Device
layout	O
where	O
another	O
key	O
such	O
as	O
the	O
right-hand	O
or	O
is	O
mapped	O
to	O
the	O
compose	B-Device
key	I-Device
,	O
this	O
option	O
is	O
normally	O
available	O
in	O
the	O
settings	O
of	O
the	O
desktop	O
environment	O
.	O
</s>
<s>
The	O
X	B-Operating_System
keyboard	B-Device
driver	O
does	O
not	O
allow	O
the	O
key	O
used	O
for	O
Compose	O
to	O
also	O
function	O
as	O
a	O
modifier	O
.	O
</s>
<s>
On	O
modern	O
systems	O
a	O
vast	O
number	O
of	O
compose	B-Device
sequences	I-Device
are	O
supported	O
.	O
</s>
<s>
On	O
Microsoft	B-Application
Windows	I-Application
,	O
a	O
few	O
programs	O
such	O
as	O
PuTTY	B-Operating_System
provide	O
compose-key	O
support	O
.	O
</s>
<s>
To	O
emulate	O
the	O
compose	B-Device
key	I-Device
for	O
all	O
software	O
,	O
keyboard	B-Device
shortcut	O
utilities	B-Application
are	O
often	O
involved	O
.	O
</s>
<s>
There	O
are	O
also	O
a	O
number	O
of	O
open-source	O
utilities	B-Application
(	O
such	O
as	O
WinCompose	O
,	O
AllChars	O
,	O
Compose-Keys	O
,	O
or	O
Compose	O
)	O
.	O
</s>
<s>
Installable	O
keyboard	B-Device
layouts	O
(	O
such	O
as	O
KbdEdit	O
)	O
are	O
available	O
that	O
contain	O
a	O
compose	B-Device
key	I-Device
assigned	O
to	O
one	O
of	O
the	O
keys	O
like	O
or	O
.	O
</s>
<s>
Such	O
keyboard	B-Device
layouts	O
can	O
also	O
be	O
programmed	O
directly	O
in	O
C	O
(	O
the	O
language	O
Windows	B-Application
drivers	O
are	O
written	O
in	O
)	O
,	O
compiled	O
using	O
the	O
free-of-charge	O
Windows	B-Application
Driver	O
Kit	O
,	O
and	O
packaged	O
using	O
the	O
free-of-charge	O
Microsoft	O
Keyboard	B-Device
Layout	O
Creator	O
1.4	O
,	O
compatible	O
up	O
to	O
the	O
latest	O
versions	O
of	O
the	O
OS	O
.	O
</s>
<s>
Although	O
the	O
Cocoa	B-Operating_System
text	O
input	O
system	O
allows	O
entry	O
of	O
many	O
alternate	O
and	O
accented	O
characters	O
natively	O
in	O
macOS	B-Application
,	O
a	O
true	O
compose-key	O
solution	O
is	O
not	O
built	O
in	O
.	O
</s>
<s>
At	O
least	O
one	O
has	O
been	O
implemented	O
using	O
the	O
Karabiner	O
utility	O
,	O
which	O
works	O
with	O
all	O
applications	O
,	O
as	O
does	O
the	O
use	O
of	O
certain	O
keyboard	B-Device
layouts	O
(	O
e.g.	O
</s>
<s>
"	O
ABC	O
–	O
Extended	O
"	O
)	O
where	O
Compose	O
is	O
implemented	O
using	O
the	O
dead	B-Device
key	I-Device
chaining	O
feature	O
.	O
</s>
<s>
Although	O
ChromeOS	B-Operating_System
is	O
supplied	O
with	O
a	O
larger	O
repertoire	O
of	O
glyphs	O
than	O
most	O
competitors	O
,	O
the	O
chords	O
needed	O
to	O
achieve	O
them	O
are	O
not	O
always	O
as	O
obvious	O
as	O
the	O
Compose	O
concept	O
provides	O
.	O
</s>
<s>
Under	O
DOS	O
,	O
compose	B-Device
key	I-Device
support	O
depended	O
on	O
the	O
running	O
application	O
,	O
or	O
on	O
a	O
loadable	O
keyboard	B-Device
driver	O
.	O
</s>
<s>
For	O
example	O
,	O
Lotus	B-Application
1-2-3	I-Application
used	O
as	O
compose	B-Device
key	I-Device
to	O
allow	O
easier	O
input	O
of	O
many	O
special	O
characters	O
of	O
the	O
Lotus	O
International	O
Character	O
Set	O
(	O
LICS	O
)	O
and	O
Lotus	B-Operating_System
Multi-Byte	I-Operating_System
Character	I-Operating_System
Set	I-Operating_System
(	O
LMBCS	B-Operating_System
)	O
.	O
</s>
<s>
The	O
table	O
shown	O
below	O
shows	O
some	O
of	O
the	O
default	O
compositions	O
for	O
the	O
X.Org	B-Operating_System
server	I-Operating_System
.	O
</s>
