<s>
In	O
information	O
systems	O
,	O
a	O
tag	B-General_Concept
is	O
a	O
keyword	O
or	O
term	O
assigned	O
to	O
a	O
piece	O
of	O
information	O
(	O
such	O
as	O
an	O
Internet	O
bookmark	O
,	O
multimedia	O
,	O
database	O
record	O
,	O
or	O
computer	B-Operating_System
file	I-Operating_System
)	O
.	O
</s>
<s>
Tags	B-General_Concept
are	O
generally	O
chosen	O
informally	O
and	O
personally	O
by	O
the	O
item	O
's	O
creator	O
or	O
by	O
its	O
viewer	O
,	O
depending	O
on	O
the	O
system	O
,	O
although	O
they	O
may	O
also	O
be	O
chosen	O
from	O
a	O
controlled	O
vocabulary	O
.	O
</s>
<s>
It	O
is	O
now	O
also	O
part	O
of	O
other	O
database	B-Application
systems	I-Application
,	O
desktop	B-Application
applications	I-Application
,	O
and	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
People	O
use	O
tags	B-General_Concept
to	O
aid	O
classification	B-General_Concept
,	O
mark	O
ownership	O
,	O
note	O
boundaries	O
,	O
and	O
indicate	O
online	B-Application
identity	I-Application
.	O
</s>
<s>
Tags	B-General_Concept
may	O
take	O
the	O
form	O
of	O
words	O
,	O
images	O
,	O
or	O
other	O
identifying	O
marks	O
.	O
</s>
<s>
An	O
analogous	O
example	O
of	O
tags	B-General_Concept
in	O
the	O
physical	O
world	O
is	O
museum	O
object	O
tagging	O
.	O
</s>
<s>
People	O
were	O
using	O
textual	O
keywords	O
to	O
classify	B-Algorithm
information	I-Algorithm
and	O
objects	O
long	O
before	O
computers	O
.	O
</s>
<s>
Computer	O
based	O
search	B-Application
algorithms	I-Application
made	O
the	O
use	O
of	O
such	O
keywords	O
a	O
rapid	O
way	O
of	O
exploring	O
records	O
.	O
</s>
<s>
Tagging	O
gained	O
popularity	O
due	O
to	O
the	O
growth	O
of	O
social	B-General_Concept
bookmarking	I-General_Concept
,	O
image	B-Algorithm
sharing	I-Algorithm
,	O
and	O
social	O
networking	O
websites	O
.	O
</s>
<s>
These	O
sites	O
allow	O
users	O
to	O
create	O
and	O
manage	O
labels	O
(	O
or	O
"	O
tags	B-General_Concept
"	O
)	O
that	O
categorize	O
content	O
using	O
simple	O
keywords	O
.	O
</s>
<s>
Websites	O
that	O
include	O
tags	B-General_Concept
often	O
display	O
collections	O
of	O
tags	B-General_Concept
as	O
tag	B-Application
clouds	I-Application
,	O
as	O
do	O
some	O
desktop	B-Application
applications	I-Application
.	O
</s>
<s>
On	O
websites	O
that	O
aggregate	O
the	O
tags	B-General_Concept
of	O
all	O
users	O
,	O
an	O
individual	O
user	O
's	O
tags	B-General_Concept
can	O
be	O
useful	O
both	O
to	O
them	O
and	O
to	O
the	O
larger	O
community	O
of	O
the	O
website	O
's	O
users	O
.	O
</s>
<s>
Top-down	O
taxonomies	O
are	O
created	O
by	O
an	O
authorized	O
group	O
of	O
designers	O
(	O
sometimes	O
in	O
the	O
form	O
of	O
a	O
controlled	O
vocabulary	O
)	O
,	O
whereas	O
bottom-up	O
taxonomies	O
(	O
called	O
folksonomies	B-General_Concept
)	O
are	O
created	O
by	O
all	O
users	O
.	O
</s>
<s>
This	O
definition	O
of	O
"	O
top	O
down	O
"	O
and	O
"	O
bottom	O
up	O
"	O
should	O
not	O
be	O
confused	O
with	O
the	O
distinction	O
between	O
a	O
single	O
hierarchical	B-Data_Structure
tree	I-Data_Structure
structure	I-Data_Structure
(	O
in	O
which	O
there	O
is	O
one	O
correct	O
way	O
to	O
classify	O
each	O
item	O
)	O
versus	O
multiple	O
non-hierarchical	O
sets	O
(	O
in	O
which	O
there	O
are	O
multiple	O
ways	O
to	O
classify	O
an	O
item	O
)	O
;	O
the	O
structure	O
of	O
both	O
top-down	O
and	O
bottom-up	O
taxonomies	O
may	O
be	O
either	O
hierarchical	O
,	O
non-hierarchical	O
,	O
or	O
a	O
combination	O
of	O
both	O
.	O
</s>
<s>
Others	O
are	O
combining	O
top-down	O
and	O
bottom-up	O
tagging	O
,	O
including	O
in	O
some	O
large	O
library	O
catalogs	O
(	O
OPACs	O
)	O
such	O
as	O
WorldCat	B-Library
.	O
</s>
<s>
When	O
tags	B-General_Concept
or	O
other	O
taxonomies	O
have	O
further	O
properties	O
(	O
or	O
semantics	B-Application
)	O
such	O
as	O
relationships	O
and	O
attributes	O
,	O
they	O
constitute	O
an	O
ontology	O
.	O
</s>
<s>
Metadata	B-General_Concept
tags	I-General_Concept
as	O
described	O
in	O
this	O
article	O
should	O
not	O
be	O
confused	O
with	O
the	O
use	O
of	O
the	O
word	O
"	O
tag	B-General_Concept
"	O
in	O
some	O
software	O
to	O
refer	O
to	O
an	O
automatically	O
generated	O
cross-reference	O
;	O
examples	O
of	O
the	O
latter	O
are	O
tags	B-General_Concept
tables	O
in	O
Emacs	B-Application
and	O
smart	B-Application
tags	I-Application
in	O
Microsoft	B-Application
Office	I-Application
.	O
</s>
<s>
The	O
use	O
of	O
keywords	O
as	O
part	O
of	O
an	O
identification	O
and	O
classification	B-General_Concept
system	O
long	O
predates	O
computers	O
.	O
</s>
<s>
Paper	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
devices	O
,	O
notably	O
edge-notched	O
cards	O
,	O
that	O
permitted	O
classification	B-General_Concept
and	O
sorting	O
by	O
multiple	O
criteria	O
were	O
already	O
in	O
use	O
prior	O
to	O
the	O
twentieth	O
century	O
,	O
and	O
faceted	O
classification	B-General_Concept
has	O
been	O
used	O
by	O
libraries	O
since	O
the	O
1930s	O
.	O
</s>
<s>
In	O
the	O
late	O
1970s	O
and	O
early	O
1980s	O
,	O
the	O
Unix	B-Application
text	B-Application
editor	I-Application
Emacs	B-Application
offered	O
a	O
companion	O
software	O
program	O
called	O
Tags	B-General_Concept
that	O
could	O
automatically	O
build	O
a	O
table	O
of	O
cross-references	O
called	O
a	O
tags	B-General_Concept
table	O
that	O
Emacs	B-Application
could	O
use	O
to	O
jump	O
between	O
a	O
function	O
call	O
and	O
that	O
function	O
's	O
definition	O
.	O
</s>
<s>
This	O
use	O
of	O
the	O
word	O
"	O
tag	B-General_Concept
"	O
did	O
not	O
refer	O
to	O
metadata	B-General_Concept
tags	I-General_Concept
,	O
but	O
was	O
an	O
early	O
use	O
of	O
the	O
word	O
"	O
tag	B-General_Concept
"	O
in	O
software	O
to	O
refer	O
to	O
a	O
word	O
index	O
.	O
</s>
<s>
Online	O
databases	B-Application
and	O
early	O
websites	O
deployed	O
keyword	O
tags	B-General_Concept
as	O
a	O
way	O
for	O
publishers	O
to	O
help	O
users	O
find	O
content	O
.	O
</s>
<s>
In	O
the	O
early	O
days	O
of	O
the	O
World	O
Wide	O
Web	O
,	O
the	O
keywords	O
meta	O
element	O
was	O
used	O
by	O
web	O
designers	O
to	O
tell	O
web	B-Application
search	I-Application
engines	I-Application
what	O
the	O
web	O
page	O
was	O
about	O
,	O
but	O
these	O
keywords	O
were	O
only	O
visible	O
in	O
a	O
web	O
page	O
's	O
source	O
code	O
and	O
were	O
not	O
modifiable	O
by	O
users	O
.	O
</s>
<s>
In	O
1997	O
,	O
the	O
collaborative	O
portal	O
"	O
A	O
Description	O
of	O
the	O
Equator	O
and	O
Some	O
ØtherLands	O
"	O
produced	O
by	O
documenta	O
X	O
,	O
Germany	O
,	O
used	O
the	O
folksonomic	B-General_Concept
term	O
Tag	B-General_Concept
for	O
its	O
co-authors	O
and	O
guest	O
authors	O
on	O
its	O
Upload	O
page	O
.	O
</s>
<s>
In	O
"	O
The	O
Equator	O
"	O
the	O
term	O
Tag	B-General_Concept
for	O
user-input	O
was	O
described	O
as	O
an	O
abstract	O
literal	O
or	O
keyword	O
to	O
aid	O
the	O
user	O
.	O
</s>
<s>
However	O
,	O
users	O
defined	O
singular	O
Tags	B-General_Concept
,	O
and	O
did	O
not	O
share	O
Tags	B-General_Concept
at	O
that	O
point	O
.	O
</s>
<s>
In	O
2003	O
,	O
the	O
social	B-General_Concept
bookmarking	I-General_Concept
website	I-General_Concept
Delicious	B-Application
provided	O
a	O
way	O
for	O
its	O
users	O
to	O
add	O
"	O
tags	B-General_Concept
"	O
to	O
their	O
bookmarks	O
(	O
as	O
a	O
way	O
to	O
help	O
find	O
them	O
later	O
)	O
;	O
Delicious	B-Application
also	O
provided	O
browseable	O
aggregated	O
views	O
of	O
the	O
bookmarks	O
of	O
all	O
users	O
featuring	O
a	O
particular	O
tag	B-General_Concept
.	O
</s>
<s>
Within	O
a	O
couple	O
of	O
years	O
,	O
the	O
photo	B-Algorithm
sharing	I-Algorithm
website	O
Flickr	B-Algorithm
allowed	O
its	O
users	O
to	O
add	O
their	O
own	O
text	O
tags	B-General_Concept
to	O
each	O
of	O
their	O
pictures	O
,	O
constructing	O
flexible	O
and	O
easy	O
metadata	O
that	O
made	O
the	O
pictures	O
highly	O
searchable	B-Application
.	O
</s>
<s>
The	O
success	O
of	O
Flickr	B-Algorithm
and	O
the	O
influence	O
of	O
Delicious	B-Application
popularized	O
the	O
concept	O
,	O
and	O
other	O
social	O
software	O
websites	O
—	O
such	O
as	O
YouTube	B-General_Concept
,	O
Technorati	B-Application
,	O
and	O
Last.fm—also	O
implemented	O
tagging	O
.	O
</s>
<s>
In	O
2005	O
,	O
the	O
Atom	B-Application
web	O
syndication	O
standard	O
provided	O
a	O
"	O
category	O
"	O
element	O
for	O
inserting	O
subject	O
categories	O
into	O
web	B-General_Concept
feeds	I-General_Concept
,	O
and	O
in	O
2007	O
Tim	O
Bray	O
proposed	O
a	O
"	O
tag	B-General_Concept
"	O
URN	B-Protocol
.	O
</s>
<s>
Many	O
systems	O
(	O
and	O
other	O
web	O
content	B-Protocol
management	I-Protocol
systems	I-Protocol
)	O
allow	O
authors	O
to	O
add	O
free-form	O
tags	B-General_Concept
to	O
a	O
post	O
,	O
along	O
with	O
(	O
or	O
instead	O
of	O
)	O
placing	O
the	O
post	O
into	O
a	O
predetermined	O
category	O
.	O
</s>
<s>
Each	O
of	O
those	O
tags	B-General_Concept
is	O
usually	O
a	O
web	O
link	O
leading	O
to	O
an	O
index	O
page	O
listing	O
all	O
of	O
the	O
posts	O
associated	O
with	O
that	O
tag	B-General_Concept
.	O
</s>
<s>
The	O
blog	O
may	O
have	O
a	O
sidebar	O
listing	O
all	O
the	O
tags	B-General_Concept
in	O
use	O
on	O
that	O
blog	O
,	O
with	O
each	O
tag	B-General_Concept
leading	O
to	O
an	O
index	O
page	O
.	O
</s>
<s>
To	O
reclassify	O
a	O
post	O
,	O
an	O
author	O
edits	O
its	O
list	O
of	O
tags	B-General_Concept
.	O
</s>
<s>
Some	O
desktop	B-Application
applications	I-Application
and	O
web	B-Application
applications	I-Application
feature	O
their	O
own	O
tagging	O
systems	O
,	O
such	O
as	O
email	O
tagging	O
in	O
Gmail	B-Application
and	O
Mozilla	B-Application
Thunderbird	I-Application
,	O
bookmark	O
tagging	O
in	O
Firefox	B-Application
,	O
audio	O
tagging	O
in	O
iTunes	B-Operating_System
or	O
Winamp	B-Application
,	O
and	O
photo	O
tagging	O
in	O
various	O
applications	O
.	O
</s>
<s>
Some	O
of	O
these	O
applications	O
display	O
collections	O
of	O
tags	B-General_Concept
as	O
tag	B-Application
clouds	I-Application
.	O
</s>
<s>
There	O
are	O
various	O
systems	O
for	O
applying	O
tags	B-General_Concept
to	O
the	O
files	O
in	O
a	O
computer	O
's	O
file	B-Application
system	I-Application
.	O
</s>
<s>
In	O
Apple	O
's	O
Mac	B-Operating_System
System	I-Operating_System
7	I-Operating_System
,	O
released	O
in	O
1991	O
,	O
users	O
could	O
assign	O
one	O
of	O
seven	B-Application
editable	I-Application
colored	I-Application
labels	I-Application
(	O
with	O
editable	O
names	O
such	O
as	O
"	O
Essential	O
"	O
,	O
"	O
Hot	O
"	O
,	O
and	O
"	O
In	O
Progress	O
"	O
)	O
to	O
each	O
file	O
and	O
folder	O
.	O
</s>
<s>
In	O
later	O
iterations	O
of	O
the	O
Mac	B-Device
operating	B-General_Concept
system	I-General_Concept
ever	O
since	O
OS	B-Operating_System
X	I-Operating_System
10.9	I-Operating_System
was	O
released	O
in	O
2013	O
,	O
users	O
could	O
assign	O
multiple	O
arbitrary	O
tags	B-General_Concept
as	O
extended	B-Application
file	I-Application
attributes	I-Application
to	O
any	O
file	O
or	O
folder	O
,	O
and	O
before	O
that	O
time	O
the	O
open-source	O
OpenMeta	O
standard	O
provided	O
similar	O
tagging	O
functionality	O
for	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
.	O
</s>
<s>
Several	O
semantic	B-Application
file	I-Application
systems	I-Application
that	O
implement	O
tags	B-General_Concept
are	O
available	O
for	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
including	O
Tagsistant	B-Application
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
allows	O
users	O
to	O
set	O
tags	B-General_Concept
only	O
on	O
Microsoft	B-Application
Office	I-Application
documents	O
and	O
some	O
kinds	O
of	O
picture	O
files	O
.	O
</s>
<s>
Cross-platform	B-Operating_System
file	O
tagging	O
standards	O
include	O
Extensible	O
Metadata	O
Platform	O
(	O
XMP	O
)	O
,	O
an	O
ISO	O
standard	O
for	O
embedding	O
metadata	O
into	O
popular	O
image	O
,	O
video	O
and	O
document	O
file	O
formats	O
,	O
such	O
as	O
JPEG	O
and	O
PDF	B-Application
,	O
without	O
breaking	O
their	O
readability	O
by	O
applications	O
that	O
do	O
not	O
support	O
XMP	O
.	O
</s>
<s>
Exif	O
is	O
a	O
standard	O
that	O
specifies	O
the	O
image	O
and	O
audio	O
file	O
formats	O
used	O
by	O
digital	B-Device
cameras	I-Device
,	O
including	O
some	O
metadata	B-General_Concept
tags	I-General_Concept
.	O
</s>
<s>
TagSpaces	B-Operating_System
is	O
an	O
open-source	O
cross-platform	B-Operating_System
application	O
for	O
tagging	O
files	O
;	O
it	O
inserts	O
tags	B-General_Concept
into	O
the	O
filename	B-Device
.	O
</s>
<s>
An	O
official	O
tag	B-General_Concept
is	O
a	O
keyword	O
adopted	O
by	O
events	O
and	O
conferences	O
for	O
participants	O
to	O
use	O
in	O
their	O
web	O
publications	O
,	O
such	O
as	O
blog	O
entries	O
,	O
photos	O
of	O
the	O
event	O
,	O
and	O
presentation	O
slides	O
.	O
</s>
<s>
Search	B-Application
engines	I-Application
can	O
then	O
index	O
them	O
to	O
make	O
relevant	O
materials	O
related	O
to	O
the	O
event	O
searchable	B-Application
in	O
a	O
uniform	O
way	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
tag	B-General_Concept
is	O
part	O
of	O
a	O
controlled	O
vocabulary	O
.	O
</s>
<s>
to	O
chapters	O
of	O
a	O
book	O
,	O
or	O
to	O
sub-themes	O
of	O
the	O
overall	O
subject	O
)	O
,	O
then	O
a	O
group	O
of	O
tags	B-General_Concept
for	O
these	O
themes	O
can	O
be	O
attached	O
to	O
each	O
of	O
the	O
items	O
in	O
the	O
larger	O
collection	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
freeform	O
classification	B-General_Concept
allows	O
the	O
author	O
to	O
manage	O
what	O
would	O
otherwise	O
be	O
unwieldy	O
amounts	O
of	O
information	O
.	O
</s>
<s>
A	O
triple	O
tag	B-General_Concept
or	O
machine	O
tag	B-General_Concept
uses	O
a	O
special	O
syntax	B-Application
to	O
define	O
extra	O
semantic	O
information	O
about	O
the	O
tag	B-General_Concept
,	O
making	O
it	O
easier	O
or	O
more	O
meaningful	O
for	O
interpretation	O
by	O
a	O
computer	O
program	O
.	O
</s>
<s>
Triple	O
tags	B-General_Concept
comprise	O
three	O
parts	O
:	O
a	O
namespace	O
,	O
a	O
predicate	O
,	O
and	O
a	O
value	O
.	O
</s>
<s>
For	O
example	O
,	O
geo:long	O
=	O
50.123456	O
is	O
a	O
tag	B-General_Concept
for	O
the	O
geographical	O
longitude	O
coordinate	O
whose	O
value	O
is	O
50.123456	O
.	O
</s>
<s>
The	O
triple	O
tag	B-General_Concept
format	O
was	O
first	O
devised	O
for	O
geolicious	O
in	O
November	O
2004	O
,	O
to	O
map	O
Delicious	B-Application
bookmarks	O
,	O
and	O
gained	O
wider	O
acceptance	O
after	O
its	O
adoption	O
by	O
Mappr	O
and	O
GeoBloggers	O
to	O
map	O
Flickr	B-Algorithm
photos	O
.	O
</s>
<s>
In	O
January	O
2007	O
,	O
Aaron	O
Straup	O
Cope	O
at	O
Flickr	B-Algorithm
introduced	O
the	O
term	O
machine	O
tag	B-General_Concept
as	O
an	O
alternative	O
name	O
for	O
the	O
triple	O
tag	B-General_Concept
,	O
adding	O
some	O
questions	O
and	O
answers	O
on	O
purpose	O
,	O
syntax	B-Application
,	O
and	O
use	O
.	O
</s>
<s>
Specialized	O
metadata	O
for	O
geographical	O
identification	O
is	O
known	O
as	O
geotagging	O
;	O
machine	O
tags	B-General_Concept
are	O
also	O
used	O
for	O
other	O
purposes	O
,	O
such	O
as	O
identifying	O
photos	O
taken	O
at	O
a	O
specific	O
event	O
or	O
naming	O
species	O
using	O
binomial	O
nomenclature	O
.	O
</s>
<s>
A	O
hashtag	O
is	O
a	O
kind	O
of	O
metadata	B-General_Concept
tag	I-General_Concept
marked	O
by	O
the	O
prefix	O
#	O
,	O
sometimes	O
known	O
as	O
a	O
"	O
hash	O
"	O
symbol	O
.	O
</s>
<s>
This	O
form	O
of	O
tagging	O
is	O
used	O
on	O
microblogging	B-Application
and	O
social	O
networking	O
services	O
such	O
as	O
Twitter	B-Application
,	O
Facebook	B-Application
,	O
Google+	B-Application
,	O
VK	O
and	O
Instagram	B-Application
.	O
</s>
<s>
The	O
hash	O
is	O
used	O
to	O
distinguish	O
tag	B-General_Concept
text	O
,	O
as	O
distinct	O
,	O
from	O
other	O
text	O
in	O
the	O
post	O
.	O
</s>
<s>
A	O
knowledge	O
tag	B-General_Concept
is	O
a	O
type	O
of	O
meta-information	O
that	O
describes	O
or	O
defines	O
some	O
aspect	O
of	O
a	O
piece	O
of	O
information	O
(	O
such	O
as	O
a	O
document	O
,	O
digital	B-Algorithm
image	I-Algorithm
,	O
database	B-Application
table	I-Application
,	O
or	O
web	O
page	O
)	O
.	O
</s>
<s>
Knowledge	B-General_Concept
tags	I-General_Concept
are	O
more	O
than	O
traditional	O
non-hierarchical	O
keywords	O
or	O
terms	O
;	O
they	O
are	O
a	O
type	O
of	O
metadata	O
that	O
captures	O
knowledge	O
in	O
the	O
form	O
of	O
descriptions	O
,	O
categorizations	O
,	O
classifications	O
,	O
semantics	B-Application
,	O
comments	O
,	O
notes	O
,	O
annotations	O
,	O
hyperdata	O
,	O
hyperlinks	O
,	O
or	O
references	O
that	O
are	O
collected	O
in	O
tag	B-General_Concept
profiles	O
(	O
a	O
kind	O
of	O
ontology	O
)	O
.	O
</s>
<s>
These	O
tag	B-General_Concept
profiles	O
reference	O
an	O
information	O
resource	O
that	O
resides	O
in	O
a	O
distributed	O
,	O
and	O
often	O
heterogeneous	O
,	O
storage	O
repository	O
.	O
</s>
<s>
Knowledge	B-General_Concept
tags	I-General_Concept
are	O
part	O
of	O
a	O
knowledge	O
management	O
discipline	O
that	O
leverages	O
Enterprise	O
2.0	O
methodologies	O
for	O
users	O
to	O
capture	O
insights	O
,	O
expertise	O
,	O
attributes	O
,	O
dependencies	O
,	O
or	O
relationships	O
associated	O
with	O
a	O
data	O
resource	O
.	O
</s>
<s>
Different	O
kinds	O
of	O
knowledge	O
can	O
be	O
captured	O
in	O
knowledge	B-General_Concept
tags	I-General_Concept
,	O
including	O
factual	O
knowledge	O
(	O
that	O
found	O
in	O
books	O
and	O
data	O
)	O
,	O
conceptual	O
knowledge	O
(	O
found	O
in	O
perspectives	O
and	O
concepts	O
)	O
,	O
expectational	O
knowledge	O
(	O
needed	O
to	O
make	O
judgments	O
and	O
hypothesis	O
)	O
,	O
and	O
methodological	O
knowledge	O
(	O
derived	O
from	O
reasoning	O
and	O
strategies	O
)	O
.	O
</s>
<s>
Knowledge	B-General_Concept
tags	I-General_Concept
are	O
considered	O
an	O
expansion	O
of	O
the	O
information	O
itself	O
that	O
adds	O
additional	O
value	O
,	O
context	O
,	O
and	O
meaning	O
to	O
the	O
information	O
.	O
</s>
<s>
Knowledge	B-General_Concept
tags	I-General_Concept
are	O
valuable	O
for	O
preserving	O
organizational	O
intelligence	O
that	O
is	O
often	O
lost	O
due	O
to	O
turnover	O
,	O
for	O
sharing	O
knowledge	O
stored	O
in	O
the	O
minds	O
of	O
individuals	O
that	O
is	O
typically	O
isolated	O
and	O
unharnessed	O
by	O
the	O
organization	O
,	O
and	O
for	O
connecting	O
knowledge	O
that	O
is	O
often	O
lost	O
or	O
disconnected	O
from	O
an	O
information	O
resource	O
.	O
</s>
<s>
In	O
a	O
typical	O
tagging	O
system	O
,	O
there	O
is	O
no	O
explicit	O
information	O
about	O
the	O
meaning	O
or	O
semantics	B-Application
of	O
each	O
tag	B-General_Concept
,	O
and	O
a	O
user	O
can	O
apply	O
new	O
tags	B-General_Concept
to	O
an	O
item	O
as	O
easily	O
as	O
applying	O
older	O
tags	B-General_Concept
.	O
</s>
<s>
Hierarchical	O
classification	B-General_Concept
systems	O
can	O
be	O
slow	O
to	O
change	O
,	O
and	O
are	O
rooted	O
in	O
the	O
culture	O
and	O
era	O
that	O
created	O
them	O
;	O
in	O
contrast	O
,	O
the	O
flexibility	O
of	O
tagging	O
allows	O
users	O
to	O
classify	O
their	O
collections	O
of	O
items	O
in	O
the	O
ways	O
that	O
they	O
find	O
useful	O
,	O
but	O
the	O
personalized	O
variety	O
of	O
terms	O
can	O
present	O
challenges	O
when	O
searching	O
and	O
browsing	O
.	O
</s>
<s>
When	O
users	O
can	O
freely	O
choose	O
tags	B-General_Concept
(	O
creating	O
a	O
folksonomy	B-General_Concept
,	O
as	O
opposed	O
to	O
selecting	O
terms	O
from	O
a	O
controlled	O
vocabulary	O
)	O
,	O
the	O
resulting	O
metadata	O
can	O
include	O
homonyms	O
(	O
the	O
same	O
tags	B-General_Concept
used	O
with	O
different	O
meanings	O
)	O
and	O
synonyms	B-Application
(	O
multiple	O
tags	B-General_Concept
for	O
the	O
same	O
concept	O
)	O
,	O
which	O
may	O
lead	O
to	O
inappropriate	O
connections	O
between	O
items	O
and	O
inefficient	O
searches	O
for	O
information	O
about	O
a	O
subject	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
tag	B-General_Concept
"	O
orange	O
"	O
may	O
refer	O
to	O
the	O
fruit	O
or	O
the	O
color	O
,	O
and	O
items	O
related	O
to	O
a	O
version	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
may	O
be	O
tagged	O
"	O
Linux	B-Operating_System
"	O
,	O
"	O
kernel	B-Operating_System
"	O
,	O
"	O
Penguin	O
"	O
,	O
"	O
software	O
"	O
,	O
or	O
a	O
variety	O
of	O
other	O
terms	O
.	O
</s>
<s>
Users	O
can	O
also	O
choose	O
tags	B-General_Concept
that	O
are	O
different	O
inflections	O
of	O
words	O
(	O
such	O
as	O
singular	O
and	O
plural	O
)	O
,	O
which	O
can	O
contribute	O
to	O
navigation	O
difficulties	O
if	O
the	O
system	O
does	O
not	O
include	O
stemming	B-General_Concept
of	O
tags	B-General_Concept
when	O
searching	O
or	O
browsing	O
.	O
</s>
<s>
Larger-scale	O
folksonomies	B-General_Concept
address	O
some	O
of	O
the	O
problems	O
of	O
tagging	O
,	O
in	O
that	O
users	O
of	O
tagging	O
systems	O
tend	O
to	O
notice	O
the	O
current	O
use	O
of	O
"	O
tag	B-General_Concept
terms	O
"	O
within	O
these	O
systems	O
,	O
and	O
thus	O
use	O
existing	O
tags	B-General_Concept
in	O
order	O
to	O
easily	O
form	O
connections	O
to	O
related	O
items	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
folksonomies	B-General_Concept
may	O
collectively	O
develop	O
a	O
partial	O
set	O
of	O
tagging	O
conventions	O
.	O
</s>
<s>
Despite	O
the	O
apparent	O
lack	O
of	O
control	O
,	O
research	O
has	O
shown	O
that	O
a	O
simple	O
form	O
of	O
shared	O
vocabulary	O
emerges	O
in	O
social	B-General_Concept
bookmarking	I-General_Concept
systems	O
.	O
</s>
<s>
Collaborative	B-General_Concept
tagging	I-General_Concept
exhibits	O
a	O
form	O
of	O
complex	O
systems	O
dynamics	O
(	O
or	O
self-organizing	O
dynamics	O
)	O
.	O
</s>
<s>
Thus	O
,	O
even	O
if	O
no	O
central	O
controlled	O
vocabulary	O
constrains	O
the	O
actions	O
of	O
individual	O
users	O
,	O
the	O
distribution	O
of	O
tags	B-General_Concept
converges	O
over	O
time	O
to	O
stable	O
power	O
law	O
distributions	O
.	O
</s>
<s>
Once	O
such	O
stable	O
distributions	O
form	O
,	O
simple	O
folksonomic	B-General_Concept
vocabularies	O
can	O
be	O
extracted	O
by	O
examining	O
the	O
correlations	O
that	O
form	O
between	O
different	O
tags	B-General_Concept
.	O
</s>
<s>
In	O
addition	O
,	O
research	O
has	O
suggested	O
that	O
it	O
is	O
easier	O
for	O
machine	O
learning	O
algorithms	O
to	O
learn	O
tag	B-General_Concept
semantics	B-Application
when	O
users	O
tag	B-General_Concept
"	O
verbosely	O
"	O
—	O
when	O
they	O
annotate	O
resources	O
with	O
a	O
wealth	O
of	O
freely	O
associated	O
,	O
descriptive	O
keywords	O
.	O
</s>
<s>
Tagging	O
systems	O
open	O
to	O
the	O
public	O
are	O
also	O
open	O
to	O
tag	B-General_Concept
spam	I-General_Concept
,	O
in	O
which	O
people	O
apply	O
an	O
excessive	O
number	O
of	O
tags	B-General_Concept
or	O
unrelated	O
tags	B-General_Concept
to	O
an	O
item	O
(	O
such	O
as	O
a	O
YouTube	B-General_Concept
video	I-General_Concept
)	O
in	O
order	O
to	O
attract	O
viewers	O
.	O
</s>
<s>
The	O
number	O
of	O
tags	B-General_Concept
allowed	O
may	O
also	O
be	O
limited	O
to	O
reduce	O
spam	O
.	O
</s>
<s>
Some	O
tagging	O
systems	O
provide	O
a	O
single	O
text	O
box	O
to	O
enter	O
tags	B-General_Concept
,	O
so	O
to	O
be	O
able	O
to	O
tokenize	O
the	O
string	O
,	O
a	O
separator	O
must	O
be	O
used	O
.	O
</s>
<s>
To	O
enable	O
the	O
use	O
of	O
separators	O
in	O
the	O
tags	B-General_Concept
,	O
a	O
system	O
may	O
allow	O
for	O
higher-level	O
separators	O
(	O
such	O
as	O
quotation	O
marks	O
)	O
or	O
escape	B-Algorithm
characters	I-Algorithm
.	O
</s>
<s>
Systems	O
can	O
avoid	O
the	O
use	O
of	O
separators	O
by	O
allowing	O
only	O
one	O
tag	B-General_Concept
to	O
be	O
added	O
to	O
each	O
input	O
widget	O
at	O
a	O
time	O
,	O
although	O
this	O
makes	O
adding	O
multiple	O
tags	B-General_Concept
more	O
time-consuming	O
.	O
</s>
<s>
A	O
syntax	B-Application
for	O
use	O
within	O
HTML	B-Language
is	O
to	O
use	O
the	O
rel-tag	O
microformat	B-General_Concept
which	O
uses	O
the	O
rel	B-General_Concept
attribute	I-General_Concept
with	O
value	O
"	O
tag	B-General_Concept
"	O
(	O
i.e.	O
,	O
rel	O
=	O
"	O
tag	B-General_Concept
"	O
)	O
to	O
indicate	O
that	O
the	O
linked-to	O
page	O
acts	O
as	O
a	O
tag	B-General_Concept
for	O
the	O
current	O
context	O
.	O
</s>
