<s>
XML	B-Language
namespaces	I-Language
are	O
used	O
for	O
providing	O
uniquely	O
named	O
elements	O
and	O
attributes	O
in	O
an	O
XML	B-Protocol
document	I-Protocol
.	O
</s>
<s>
An	O
XML	B-Protocol
instance	O
may	O
contain	O
element	O
or	O
attribute	O
names	O
from	O
more	O
than	O
one	O
XML	B-Protocol
vocabulary	I-Protocol
.	O
</s>
<s>
A	O
simple	O
example	O
would	O
be	O
to	O
consider	O
an	O
XML	B-Protocol
instance	O
that	O
contained	O
references	O
to	O
a	O
customer	O
and	O
an	O
ordered	O
product	O
.	O
</s>
<s>
A	O
namespace	O
name	O
is	O
a	O
uniform	B-Protocol
resource	I-Protocol
identifier	I-Protocol
(	O
URI	B-Protocol
)	O
.	O
</s>
<s>
Typically	O
,	O
the	O
URI	B-Protocol
chosen	O
for	O
the	O
namespace	O
of	O
a	O
given	O
XML	B-Protocol
vocabulary	I-Protocol
describes	O
a	O
resource	O
under	O
the	O
control	O
of	O
the	O
author	O
or	O
organization	O
defining	O
the	O
vocabulary	O
,	O
such	O
as	O
a	O
URL	O
for	O
the	O
author	O
's	O
Web	O
server	O
.	O
</s>
<s>
However	O
,	O
the	O
namespace	O
specification	O
does	O
not	O
require	O
nor	O
suggest	O
that	O
the	O
namespace	O
URI	B-Protocol
be	O
used	O
to	O
retrieve	O
information	O
;	O
it	O
is	O
simply	O
treated	O
by	O
an	O
XML	B-Protocol
parser	I-Protocol
as	O
a	O
string	O
.	O
</s>
<s>
It	O
simply	O
describes	O
the	O
XHTML	B-Language
namespace	O
to	O
human	O
readers	O
.	O
</s>
<s>
Using	O
a	O
URI	B-Protocol
(	O
such	O
as	O
"	O
http://www.w3.org/1999/xhtml	O
"	O
)	O
to	O
identify	O
a	O
namespace	O
,	O
rather	O
than	O
a	O
simple	O
string	O
(	O
such	O
as	O
"	O
xhtml	B-Language
"	O
)	O
,	O
reduces	O
the	O
probability	O
of	O
different	O
namespaces	O
using	O
duplicate	O
identifiers	O
.	O
</s>
<s>
Although	O
the	O
term	O
namespace	O
URI	B-Protocol
is	O
widespread	O
,	O
the	O
W3C	O
Recommendation	O
refers	O
to	O
it	O
as	O
the	O
namespace	O
name	O
.	O
</s>
<s>
The	O
specification	O
is	O
not	O
entirely	O
prescriptive	O
about	O
the	O
precise	O
rules	O
for	O
namespace	O
names	O
(	O
it	O
does	O
not	O
explicitly	O
say	O
that	O
parsers	O
must	O
reject	O
documents	O
where	O
the	O
namespace	O
name	O
is	O
not	O
a	O
valid	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
)	O
,	O
and	O
many	O
XML	B-Protocol
parsers	I-Protocol
allow	O
any	O
character	O
string	O
to	O
be	O
used	O
.	O
</s>
<s>
In	O
version	O
1.1	O
of	O
the	O
recommendation	O
,	O
the	O
namespace	O
name	O
becomes	O
an	O
Internationalized	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
,	O
which	O
licenses	O
the	O
use	O
of	O
non-ASCII	O
characters	O
that	O
in	O
practice	O
were	O
already	O
accepted	O
by	O
nearly	O
all	O
XML	B-Protocol
software	O
.	O
</s>
<s>
The	O
term	O
namespace	O
URI	B-Protocol
persists	O
,	O
however	O
,	O
not	O
only	O
in	O
popular	O
usage	O
,	O
but	O
also	O
in	O
many	O
other	O
specifications	O
from	O
W3C	O
and	O
elsewhere	O
.	O
</s>
<s>
Following	O
publication	O
of	O
the	O
Namespaces	O
recommendation	O
,	O
there	O
was	O
an	O
intensive	O
debate	O
about	O
how	O
a	O
relative	O
URI	B-Protocol
should	O
be	O
handled	O
,	O
with	O
some	O
intensely	O
arguing	O
that	O
it	O
should	O
simply	O
be	O
treated	O
as	O
a	O
character	O
string	O
,	O
and	O
others	O
arguing	O
with	O
conviction	O
that	O
it	O
should	O
be	O
turned	O
into	O
an	O
absolute	O
URI	B-Protocol
by	O
resolving	O
it	O
against	O
the	O
base	O
URI	B-Protocol
of	O
the	O
document	O
.	O
</s>
<s>
The	O
result	O
of	O
the	O
debate	O
was	O
a	O
ruling	O
from	O
W3C	O
that	O
relative	O
URIs	B-Protocol
were	O
deprecated	O
.	O
</s>
<s>
The	O
use	O
of	O
URIs	B-Protocol
taking	O
the	O
form	O
of	O
URLs	O
in	O
the	O
http	O
scheme	O
(	O
such	O
as	O
http://www.w3.org/1999/xhtml	O
)	O
is	O
common	O
,	O
despite	O
the	O
absence	O
of	O
any	O
formal	O
relationship	O
with	O
the	O
HTTP	O
protocol	O
.	O
</s>
<s>
One	O
convention	O
adopted	O
by	O
some	O
users	O
is	O
to	O
place	O
an	O
RDDL	B-General_Concept
document	O
at	O
the	O
location	O
.	O
</s>
<s>
In	O
general	O
,	O
however	O
,	O
users	O
should	O
assume	O
that	O
the	O
namespace	O
URI	B-Protocol
is	O
simply	O
a	O
name	O
,	O
not	O
the	O
address	O
of	O
a	O
document	O
on	O
the	O
Web	O
.	O
</s>
<s>
An	O
XML	B-Language
namespace	I-Language
is	O
declared	O
using	O
the	O
reserved	O
XML	B-Protocol
attribute	O
xmlns	B-Language
or	O
xmlns:prefix	O
,	O
the	O
value	O
of	O
which	O
must	O
be	O
a	O
valid	O
namespace	O
name	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
following	O
declaration	O
maps	O
the	O
"	O
xhtml	B-Language
:	O
"	O
prefix	O
to	O
the	O
XHTML	B-Language
namespace	O
:	O
</s>
<s>
Any	O
element	O
or	O
attribute	O
whose	O
name	O
starts	O
with	O
the	O
prefix	O
"	O
xhtml	B-Language
:	O
"	O
is	O
considered	O
to	O
be	O
in	O
the	O
XHTML	B-Language
namespace	O
,	O
if	O
it	O
or	O
an	O
ancestor	O
has	O
the	O
above	O
namespace	O
declaration	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
any	O
element	O
without	O
a	O
namespace	O
prefix	O
is	O
considered	O
to	O
be	O
in	O
the	O
XHTML	B-Language
namespace	O
,	O
if	O
it	O
or	O
an	O
ancestor	O
has	O
the	O
above	O
default	O
namespace	O
declaration	O
.	O
</s>
<s>
Nearly	O
all	O
programming	O
models	O
allow	O
the	O
name	O
of	O
an	O
element	O
or	O
attribute	O
node	O
to	O
be	O
retrieved	O
as	O
a	O
three-part	O
name	O
:	O
the	O
local	O
name	O
,	O
the	O
namespace	O
prefix	O
,	O
and	O
the	O
namespace	O
URI	B-Protocol
.	O
</s>
<s>
Names	O
are	O
considered	O
equal	O
if	O
the	O
namespace	O
URI	B-Protocol
and	O
local	O
name	O
match	O
.	O
</s>
<s>
This	O
information	O
is	O
needed	O
because	O
some	O
XML	B-Protocol
vocabularies	I-Protocol
allow	O
qualified	O
names	O
(	O
containing	O
namespace	O
prefixes	O
)	O
to	O
appear	O
in	O
the	O
content	O
of	O
elements	O
or	O
attributes	O
,	O
as	O
well	O
as	O
in	O
their	O
names	O
.	O
</s>
<s>
As	O
attribute	O
nodes	O
named	O
"	O
xmlns	B-Language
"	O
or	O
"	O
xmlns:xxx	O
"	O
,	O
exactly	O
as	O
the	O
namespaces	O
are	O
written	O
in	O
the	O
source	O
XML	B-Protocol
document	I-Protocol
.	O
</s>
<s>
This	O
is	O
the	O
model	O
presented	O
by	O
DOM	B-General_Concept
.	O
</s>
<s>
As	O
namespace	O
declarations	O
:	O
distinguished	O
from	O
attributes	O
,	O
but	O
corresponding	O
one-to-one	O
with	O
the	O
relevant	O
attributes	O
in	O
the	O
source	O
XML	B-Protocol
document	I-Protocol
.	O
</s>
<s>
This	O
is	O
the	O
model	O
presented	O
by	O
JDOM	B-Language
.	O
</s>
<s>
This	O
is	O
the	O
model	O
used	O
in	O
XPath	B-Language
,	O
XSLT	B-Application
,	O
and	O
XQuery	B-Language
.	O
</s>
