<s>
A	O
numeric	B-General_Concept
character	I-General_Concept
reference	I-General_Concept
(	O
NCR	O
)	O
is	O
a	O
common	O
markup	O
construct	O
used	O
in	O
SGML	B-Language
and	O
SGML-derived	O
markup	O
languages	O
such	O
as	O
HTML	B-Language
and	O
XML	B-Protocol
.	O
</s>
<s>
Since	O
WebSgml	B-Language
,	O
XML	B-Protocol
and	O
HTML	B-Language
4	O
,	O
the	O
code	O
points	O
of	O
the	O
Universal	O
Character	B-General_Concept
Set	I-General_Concept
(	O
UCS	O
)	O
of	O
Unicode	O
are	O
used	O
.	O
</s>
<s>
NCRs	O
are	O
typically	O
used	O
in	O
order	O
to	O
represent	O
characters	O
that	O
are	O
not	O
directly	O
encodable	O
in	O
a	O
particular	O
document	O
(	O
for	O
example	O
,	O
because	O
they	O
are	O
international	O
characters	O
that	O
do	O
not	O
fit	O
in	O
the	O
8-bit	O
character	B-General_Concept
set	I-General_Concept
being	O
used	O
,	O
or	O
because	O
they	O
have	O
special	O
syntactic	O
meaning	O
in	O
the	O
language	O
)	O
.	O
</s>
<s>
List	O
of	O
numeric	B-General_Concept
character	I-General_Concept
references	I-General_Concept
for	O
the	O
printable	O
ASCII	B-Protocol
characters	I-Protocol
:	O
</s>
<s>
Unicode	O
character	O
CharacterReference(decimal )	O
CharacterReference(hexadecimal )	O
Effect	O
U+0020	O
(	O
space	O
)	O
U+0021	O
!	O
!	O
!	O
</s>
<s>
That	O
is	O
,	O
a	O
document	O
consists	O
,	O
at	O
its	O
most	O
fundamental	O
level	O
of	O
abstraction	O
,	O
of	O
a	O
sequence	O
of	O
characters	O
,	O
which	O
are	O
abstract	O
units	O
that	O
exist	O
independently	O
of	O
any	O
encoding	B-General_Concept
.	O
</s>
<s>
Ideally	O
,	O
when	O
the	O
characters	O
of	O
a	O
document	O
utilizing	O
a	O
markup	O
language	O
are	O
encoded	O
for	O
storage	O
or	O
transmission	O
over	O
a	O
network	O
as	O
a	O
sequence	O
of	O
bits	O
,	O
the	O
encoding	B-General_Concept
that	O
is	O
used	O
will	O
be	O
one	O
that	O
supports	O
representing	O
each	O
and	O
every	O
character	O
in	O
the	O
document	O
,	O
if	O
not	O
in	O
the	O
whole	O
of	O
Unicode	O
,	O
directly	O
as	O
a	O
particular	O
bit	O
sequence	O
.	O
</s>
<s>
Sometimes	O
,	O
though	O
,	O
for	O
reasons	O
of	O
convenience	O
or	O
due	O
to	O
technical	O
limitations	O
,	O
documents	O
are	O
encoded	O
with	O
an	O
encoding	B-General_Concept
that	O
cannot	O
represent	O
some	O
characters	O
directly	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
widely	O
used	O
encodings	O
based	O
on	O
ISO	O
8859	O
can	O
only	O
represent	O
,	O
at	O
most	O
,	O
256	O
unique	O
characters	O
as	O
one	O
8-bit	O
byte	B-Application
each	O
.	O
</s>
<s>
Documents	O
are	O
rarely	O
,	O
in	O
practice	O
,	O
ever	O
allowed	O
to	O
use	O
more	O
than	O
one	O
encoding	B-General_Concept
internally	O
,	O
so	O
the	O
onus	O
is	O
usually	O
on	O
the	O
markup	O
language	O
to	O
provide	O
a	O
means	O
for	O
document	O
authors	O
to	O
express	O
unencodable	O
characters	O
in	O
terms	O
of	O
encodable	O
ones	O
.	O
</s>
<s>
This	O
is	O
generally	O
done	O
through	O
some	O
kind	O
of	O
"	B-Algorithm
escaping	I-Algorithm
"	I-Algorithm
mechanism	I-Algorithm
.	O
</s>
<s>
The	O
SGML-based	O
markup	O
languages	O
allow	O
document	O
authors	O
to	O
use	O
special	O
sequences	O
of	O
characters	O
from	O
the	O
ASCII	B-Protocol
range	O
(	O
the	O
first	O
128	O
code	O
points	O
of	O
Unicode	O
)	O
to	O
represent	O
,	O
or	O
reference	O
,	O
any	O
Unicode	O
character	O
,	O
regardless	O
of	O
whether	O
the	O
character	O
being	O
represented	O
is	O
directly	O
available	O
in	O
the	O
document	O
's	O
encoding	B-General_Concept
.	O
</s>
<s>
Character	O
references	O
that	O
are	O
based	O
on	O
the	O
referenced	O
character	O
's	O
UCS	O
or	O
Unicode	O
code	O
point	O
are	O
called	O
numeric	B-General_Concept
character	I-General_Concept
references	I-General_Concept
.	O
</s>
<s>
In	O
HTML	B-Language
4	O
and	O
in	O
all	O
versions	O
of	O
XHTML	B-Language
and	O
XML	B-Protocol
,	O
the	O
code	O
point	O
can	O
be	O
expressed	O
either	O
as	O
a	O
decimal	B-Application
(	O
base	O
10	O
)	O
number	O
or	O
as	O
a	O
hexadecimal	O
(	O
base	O
16	O
)	O
number	O
.	O
</s>
<s>
Older	O
versions	O
of	O
HTML	B-Language
disallowed	O
the	O
hexadecimal	O
syntax	O
.	O
</s>
<s>
The	O
characters	O
that	O
comprise	O
a	O
numeric	B-General_Concept
character	I-General_Concept
reference	I-General_Concept
can	O
be	O
represented	O
in	O
every	O
character	B-General_Concept
encoding	I-General_Concept
used	O
in	O
computing	O
and	O
telecommunications	O
today	O
,	O
so	O
there	O
is	O
no	O
risk	O
of	O
the	O
reference	O
itself	O
being	O
unencodable	O
.	O
</s>
<s>
There	O
is	O
another	O
kind	O
of	O
character	O
reference	O
called	O
a	O
character	B-General_Concept
entity	I-General_Concept
reference	I-General_Concept
,	O
which	O
allows	O
a	O
character	O
to	O
be	O
referred	O
to	O
by	O
a	O
name	O
instead	O
of	O
a	O
number	O
.	O
</s>
<s>
(	O
Naming	O
a	O
character	O
creates	O
a	O
character	B-General_Concept
entity	I-General_Concept
.	O
)	O
</s>
<s>
HTML	B-Language
defines	O
some	O
character	B-General_Concept
entities	I-General_Concept
,	O
but	O
not	O
many	O
;	O
all	O
other	O
characters	O
can	O
only	O
be	O
included	O
by	O
direct	O
encoding	B-General_Concept
or	O
using	O
NCRs	O
.	O
</s>
<s>
The	O
Universal	O
Character	B-General_Concept
Set	I-General_Concept
defined	O
by	O
ISO	O
10646	O
is	O
the	O
"	O
document	O
character	B-General_Concept
set	I-General_Concept
"	O
of	O
SGML	B-Language
,	O
HTML	B-Language
4	O
,	O
so	O
by	O
default	O
,	O
any	O
character	O
in	O
such	O
a	O
document	O
,	O
and	O
any	O
character	O
referenced	O
in	O
such	O
a	O
document	O
,	O
must	O
be	O
in	O
the	O
UCS	O
.	O
</s>
<s>
While	O
the	O
syntax	O
of	O
SGML	B-Language
does	O
not	O
prohibit	O
references	O
to	O
invalid	O
or	O
unassigned	O
code	O
points	O
,	O
such	O
as	O
,	O
SGML-derived	O
markup	O
languages	O
such	O
as	O
HTML	B-Language
and	O
XML	B-Protocol
can	O
,	O
and	O
often	O
do	O
,	O
restrict	O
numeric	B-General_Concept
character	I-General_Concept
references	I-General_Concept
to	O
only	O
those	O
code	O
points	O
that	O
are	O
assigned	O
to	O
characters	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
HTML	B-Language
4	O
,	O
,	O
which	O
is	O
a	O
reference	O
to	O
a	O
non-printing	O
"	O
form	O
feed	O
"	O
control	O
character	O
,	O
is	O
allowed	O
because	O
a	O
form	O
feed	O
character	O
is	O
allowed	O
.	O
</s>
<s>
But	O
in	O
XML	B-Protocol
,	O
the	O
form	O
feed	O
character	O
cannot	O
be	O
used	O
,	O
not	O
even	O
by	O
reference	O
.	O
</s>
<s>
As	O
another	O
example	O
,	O
€	O
,	O
which	O
is	O
a	O
reference	O
to	O
another	O
control	O
character	O
,	O
is	O
not	O
allowed	O
to	O
be	O
used	O
or	O
referenced	O
in	O
either	O
HTML	B-Language
or	O
XML	B-Protocol
,	O
but	O
when	O
used	O
in	O
HTML	B-Language
,	O
it	O
is	O
usually	O
not	O
flagged	O
as	O
an	O
error	O
by	O
web	O
browsers	O
–	O
some	O
of	O
which	O
interpret	O
it	O
as	O
a	O
reference	O
to	O
the	O
character	O
represented	O
by	O
code	O
value	O
128	O
in	O
the	O
Windows-1252	O
encoding	B-General_Concept
for	O
compatibility	O
reasons	O
.	O
</s>
<s>
This	O
character	O
,	O
"	O
€	O
"	O
,	O
has	O
to	O
be	O
represented	O
as	O
€	O
in	O
a	O
standard-compliant	O
HTML	B-Language
code	I-Language
.	O
</s>
<s>
As	O
a	O
further	O
example	O
,	O
prior	O
to	O
the	O
publication	O
of	O
XML	B-Protocol
1.0	O
Second	O
Edition	O
on	O
October	O
6	O
,	O
2000	O
,	O
XML	B-Protocol
1.0	O
was	O
based	O
on	O
an	O
older	O
version	O
of	O
ISO	O
10646	O
and	O
prohibited	O
using	O
characters	O
above	O
U+FFFD	O
,	O
except	O
in	O
character	O
data	O
,	O
thus	O
making	O
a	O
reference	O
like	O
𐀀	O
(	O
U+10000	O
)	O
illegal	O
.	O
</s>
<s>
In	O
XML	B-Protocol
1.1	O
and	O
newer	O
editions	O
of	O
XML	B-Protocol
1.0	O
,	O
such	O
a	O
reference	O
is	O
allowed	O
,	O
because	O
the	O
available	O
character	O
repertoire	O
was	O
explicitly	O
extended	O
.	O
</s>
<s>
In	O
the	O
initial	O
versions	O
of	O
SGML	B-Language
and	O
HTML	B-Language
,	O
numeric	B-General_Concept
character	I-General_Concept
references	I-General_Concept
were	O
interpreted	O
in	O
relationship	O
to	O
the	O
document	O
character	B-General_Concept
encoding	I-General_Concept
,	O
rather	O
than	O
Unicode	O
.	O
</s>
<s>
For	O
Latin-script	O
documents	O
,	O
numeric	B-General_Concept
character	I-General_Concept
references	I-General_Concept
to	O
characters	O
between	O
x80	O
and	O
x9F	O
in	O
those	O
documents	O
will	O
not	O
be	O
correct	O
against	O
Unicode	O
,	O
and	O
must	O
be	O
recoded	O
.	O
</s>
<s>
HTML	B-Language
standards	O
prior	O
to	O
HTML	B-Language
4	O
supported	O
only	O
Western	O
Latin	O
script	O
documents	O
:	O
the	O
treatment	O
of	O
character	O
references	O
above	O
#7F	O
may	O
vary	O
between	O
applications	O
and	O
national	O
conventions	O
.	O
</s>
<s>
For	O
example	O
,	O
as	O
mentioned	O
above	O
,	O
the	O
correct	O
numeric	B-General_Concept
character	I-General_Concept
reference	I-General_Concept
for	O
the	O
Euro	O
sign	O
"	O
€	O
"	O
U+20AC	O
when	O
using	O
Unicode	O
is	O
decimal	B-Application
€	O
and	O
hexadecimal	O
€	O
.	O
</s>
<s>
However	O
,	O
if	O
using	O
tools	O
supporting	O
obsolete	O
implementations	O
of	O
HTML	B-Language
,	O
the	O
reference	O
€	O
(	O
Euro	O
sign	O
in	O
the	O
CP-1252	O
code	O
page	O
)	O
or	O
¤	O
(	O
Euro	O
sign	O
in	O
ISO/IEC	O
8859-15	O
)	O
may	O
work	O
.	O
</s>
<s>
As	O
another	O
example	O
,	O
if	O
some	O
text	O
was	O
created	O
originally	O
using	O
the	O
MacRoman	O
character	B-General_Concept
set	I-General_Concept
,	O
the	O
left	O
double	O
quotation	O
mark	O
will	O
be	O
represented	O
with	O
code	O
point	O
xD2	O
.	O
</s>
<s>
The	O
correct	O
numeric	B-General_Concept
character	I-General_Concept
reference	I-General_Concept
for	O
in	O
HTML	B-Language
4	O
and	O
newer	O
is	O
“	O
,	O
because	O
U+201C	O
is	O
its	O
UCS	O
code	O
.	O
</s>
<s>
In	O
some	O
systems	O
,	O
the	O
named	B-General_Concept
character	I-General_Concept
reference	I-General_Concept
“	O
may	O
also	O
be	O
available	O
.	O
</s>
