<s>
NOV	B-Application
,	O
or	O
News	O
Overview	O
,	O
is	O
a	O
widely	O
deployed	O
indexing	O
method	O
for	O
Usenet	B-Application
articles	O
,	O
also	O
found	O
in	O
some	O
Internet	O
email	O
implementations	O
.	O
</s>
<s>
Written	O
in	O
1992	O
by	O
Geoff	O
Collyer	O
,	O
NOV	B-Application
replaced	O
a	O
variety	O
of	O
incompatible	O
indexing	O
schemes	O
used	O
in	O
different	O
client	B-Protocol
programs	I-Protocol
,	O
each	O
typically	O
requiring	O
custom	O
modifications	O
to	O
each	O
news	B-Application
server	I-Application
before	O
they	O
could	O
be	O
used	O
.	O
</s>
<s>
In	O
modern	O
NNTP	B-Protocol
implementations	O
,	O
NOV	B-Application
is	O
exposed	O
as	O
the	O
and	O
related	O
commands	O
.	O
</s>
<s>
In	O
its	O
original	O
implementation	O
,	O
the	O
header	O
lines	O
of	O
each	O
incoming	O
message	O
are	O
examined	O
,	O
and	O
a	O
single	O
line	O
of	O
text	O
is	O
appended	O
to	O
the	O
overview	O
files	O
,	O
with	O
one	O
overview	O
file	O
present	O
for	O
each	O
newsgroup	B-Application
.	O
</s>
<s>
Tab	O
(	O
ASCII	B-Protocol
code	I-Protocol
9	O
)	O
characters	O
and	O
line	O
breaks	O
within	O
the	O
headers	O
are	O
converted	O
to	O
spaces	O
(	O
ASCII	B-Protocol
code	I-Protocol
32	O
)	O
,	O
and	O
the	O
header	O
fields	O
within	O
each	O
overview	O
line	O
are	O
then	O
delimited	O
by	O
tab	O
characters	O
.	O
</s>
<s>
The	O
first	O
seven	O
fields	O
in	O
a	O
NOV	B-Application
line	O
are	O
fixed	O
and	O
unlabeled	O
:	O
</s>
<s>
An	O
arbitrary	O
number	O
of	O
additional	O
fields	O
may	O
be	O
added	O
to	O
any	O
NOV	B-Application
line	O
.	O
</s>
<s>
Some	O
server	O
provide	O
a	O
schema	O
of	O
what	O
is	O
recorded	O
to	O
new	O
overview	O
lines	O
in	O
the	O
form	O
of	O
an	O
NNTP	B-Protocol
command	O
,	O
but	O
this	O
cannot	O
be	O
relied	O
upon	O
to	O
be	O
accurate	O
for	O
older	O
entries	O
.	O
</s>
<s>
In	O
practice	O
,	O
most	O
servers	O
supply	O
only	O
one	O
optional	O
field	O
,	O
the	O
contents	O
of	O
the	O
Xref	O
:	O
header	O
,	O
to	O
allow	O
crosspost	B-Application
management	O
.	O
</s>
<s>
While	O
virtually	O
all	O
modern	O
news	B-Application
server	I-Application
and	O
newsreader	B-Protocol
software	O
employs	O
NOV	B-Application
,	O
it	O
has	O
also	O
found	O
its	O
way	O
into	O
other	O
applications	O
such	O
as	O
email	O
clients	O
.	O
</s>
<s>
One	O
prominent	O
example	O
is	O
Gnus	B-Operating_System
,	O
which	O
can	O
take	O
advantage	O
of	O
overview	O
files	O
for	O
faster	O
access	O
to	O
large	O
mail	O
folders	O
.	O
</s>
<s>
Many	O
newer	O
news	B-Application
servers	I-Application
store	O
NOV	B-Application
data	O
in	O
a	O
variety	O
of	O
formats	O
.	O
</s>
