<s>
A	O
union	B-Library
catalog	I-Library
is	O
a	O
combined	O
library	B-Library
catalog	I-Library
describing	O
the	O
collections	O
of	O
a	O
number	O
of	O
libraries	O
.	O
</s>
<s>
Union	B-Library
catalogs	I-Library
have	O
been	O
created	O
in	O
a	O
range	O
of	O
media	O
,	O
including	O
book	O
format	O
,	O
microform	O
,	O
cards	O
and	O
more	O
recently	O
,	O
networked	O
electronic	O
databases	B-Application
.	O
</s>
<s>
Print	O
union	B-Library
catalogs	I-Library
are	O
typically	O
arranged	O
by	O
title	O
,	O
author	O
or	O
subject	O
(	O
often	O
employing	O
a	O
controlled	O
vocabulary	O
)	O
;	O
electronic	O
versions	O
typically	O
support	O
keyword	O
and	O
Boolean	O
queries	O
.	O
</s>
<s>
Union	B-Library
catalogs	I-Library
are	O
useful	O
to	O
librarians	B-Application
,	O
as	O
they	O
assist	O
in	O
locating	O
and	O
requesting	O
materials	O
from	O
other	O
libraries	O
through	O
interlibrary	O
loan	O
service	O
.	O
</s>
<s>
The	O
largest	O
union	B-Library
catalog	I-Library
ever	O
printed	O
is	O
the	O
American	O
National	O
Union	B-Library
Catalog	I-Library
Pre-1956	O
Imprints	O
(	O
NUC	O
)	O
,	O
completed	O
in	O
1981	O
.	O
</s>
<s>
This	O
achievement	O
has	O
since	O
been	O
superseded	O
by	O
the	O
creation	O
of	O
union	B-Library
catalogs	I-Library
in	O
the	O
form	O
of	O
electronic	O
databases	B-Application
,	O
of	O
which	O
the	O
largest	O
is	O
OCLC	O
's	O
WorldCat	B-Library
.	O
</s>
<s>
For	O
academic	O
publications	O
,	O
several	O
academic	B-Library
search	I-Library
engines	I-Library
exist	O
to	O
combine	O
the	O
open	O
data	O
provided	O
by	O
open	O
archives	O
through	O
OAI-PMH	O
,	O
as	O
well	O
as	O
records	O
from	O
publishers	O
deposited	O
in	O
CrossRef	O
and	O
other	O
sources	O
.	O
</s>
<s>
They	O
include	O
BASE	B-Application
,	O
CORE	B-Algorithm
and	O
Unpaywall	O
,	O
which	O
indexes	O
over	O
20	O
million	O
open	O
access	O
publications	O
as	O
of	O
2020	O
.	O
</s>
