<s>
Revision	B-Architecture
Control	I-Architecture
System	I-Architecture
(	O
RCS	B-Application
)	O
is	O
an	O
early	O
implementation	O
of	O
a	O
version	B-Architecture
control	I-Architecture
system	I-Architecture
(	O
VCS	B-Architecture
)	O
.	O
</s>
<s>
It	O
is	O
a	O
set	O
of	O
UNIX	B-Application
commands	O
that	O
allow	O
multiple	O
users	O
to	O
develop	O
and	O
maintain	O
program	O
code	O
or	O
documents	O
.	O
</s>
<s>
With	O
RCS	B-Application
,	O
users	O
can	O
make	O
their	O
own	O
revisions	O
of	O
a	O
document	O
,	O
commit	O
changes	O
,	O
and	O
merge	O
them	O
.	O
</s>
<s>
RCS	B-Application
was	O
originally	O
developed	O
for	O
programs	O
but	O
is	O
also	O
useful	O
for	O
text	O
documents	O
or	O
configuration	O
files	O
that	O
are	O
frequently	O
revised	O
.	O
</s>
<s>
It	O
was	O
an	O
alternative	O
tool	O
to	O
the	O
then-popular	O
Source	B-Device
Code	I-Device
Control	I-Device
System	I-Device
(	O
SCCS	O
)	O
which	O
was	O
nearly	O
the	O
first	O
version	B-Architecture
control	I-Architecture
software	I-Architecture
tool	O
(	O
developed	O
in	O
1972	O
by	O
early	O
Unix	B-Application
developers	O
)	O
.	O
</s>
<s>
RCS	B-Application
is	O
currently	O
maintained	O
by	O
the	B-Application
GNU	I-Application
Project	I-Application
.	O
</s>
<s>
An	O
innovation	O
in	O
RCS	B-Application
is	O
the	O
adoption	O
of	O
reverse	O
deltas	O
.	O
</s>
<s>
Instead	O
of	O
storing	O
every	O
revision	O
in	O
a	O
file	O
like	O
SCCS	O
does	O
with	O
interleaved	B-Protocol
deltas	I-Protocol
,	O
RCS	B-Application
stores	O
a	O
set	O
of	O
edit	O
instructions	O
to	O
go	O
back	O
to	O
an	O
earlier	O
version	O
of	O
the	O
file	O
.	O
</s>
<s>
A	O
READ_ME	O
file	O
accompanied	O
some	O
versions	O
of	O
RCS	B-Application
which	O
further	O
restricted	O
distribution	O
,	O
e.g.	O
,	O
in	O
4.3BSD-Reno	O
.	O
</s>
<s>
1989	O
,	O
the	O
RCS	B-Application
license	O
was	O
altered	O
to	O
something	O
similar	O
to	O
the	O
contemporary	O
BSD	O
licenses	O
,	O
as	O
seen	O
by	O
comments	O
in	O
the	O
source	O
code	O
.	O
</s>
<s>
RCS	B-Application
4.3	O
,	O
released	O
26	O
July	O
1990	O
,	O
was	O
distributed	O
"	O
under	O
license	O
by	O
the	B-Operating_System
Free	I-Operating_System
Software	I-Operating_System
Foundation	I-Operating_System
"	O
,	O
under	O
the	O
terms	O
of	O
the	O
GPL	B-License
.	O
</s>
<s>
RCS	B-Application
operates	O
only	O
on	O
single	O
files	O
.	O
</s>
<s>
RCS	B-Application
revolves	O
around	O
the	O
usage	O
of	O
"	O
revision	O
groups	O
"	O
or	O
sets	O
of	O
files	O
that	O
have	O
been	O
checked-in	O
via	O
the	O
co	B-Application
(	O
checkout	O
)	O
and	O
ci	B-Application
(	O
check-in	O
)	O
commands	O
.	O
</s>
<s>
By	O
default	O
,	O
a	O
checked-in	O
file	O
is	O
removed	O
and	O
replaced	O
with	O
a	O
"	O
,	O
v	B-Operating_System
"	O
file	O
(	O
so	O
foo.rb	O
when	O
checked	O
in	O
becomes	O
foo.rb	O
,	O
v	B-Operating_System
)	O
which	O
can	O
then	O
be	O
checked	O
out	O
by	O
anyone	O
with	O
access	O
to	O
the	O
revision	O
group	O
.	O
</s>
<s>
RCS	B-Application
files	O
(	O
again	O
,	O
files	O
with	O
the	O
extension	O
"	O
,	O
v	B-Operating_System
"	O
)	O
reflect	O
the	O
main	O
file	O
with	O
additional	O
metadata	O
on	O
its	O
first	O
lines	O
.	O
</s>
<s>
Once	O
checked	O
in	O
,	O
RCS	B-Application
stores	O
revisions	O
in	O
a	O
tree	O
structure	O
that	O
can	O
be	O
followed	O
so	O
that	O
a	O
user	O
can	O
revert	O
a	O
file	O
to	O
a	O
previous	O
form	O
if	O
necessary	O
.	O
</s>
<s>
SCCS	O
(	O
first	O
released	O
in	O
1973	O
)	O
and	O
DSEE	O
(	O
considered	O
a	O
predecessor	O
of	O
Atria	B-Protocol
ClearCase	I-Protocol
)	O
,	O
described	O
in	O
1984	O
,	O
are	O
two	O
other	O
notable	O
VCS	B-Architecture
software	O
tools	O
.	O
</s>
<s>
These	O
tools	O
,	O
along	O
with	O
RCS	B-Application
,	O
are	O
generally	O
considered	O
the	O
first	O
generation	O
of	O
VCS	B-Architecture
as	O
automated	O
software	O
tools	O
.	O
</s>
<s>
After	O
the	O
first	O
generation	O
VCS	B-Architecture
,	O
tools	O
such	O
as	O
CVS	B-Application
and	O
Subversion	B-Application
,	O
which	O
feature	O
a	O
locally	O
centralized	O
repository	O
,	O
could	O
be	O
considered	O
as	O
the	O
second	O
generation	O
VCS	B-Architecture
.	O
</s>
<s>
Specifically	O
,	O
CVS	B-Application
(	O
Concurrent	B-Application
Versions	I-Application
System	I-Application
)	O
was	O
developed	O
on	O
top	O
of	O
RCS	B-Application
structure	O
,	O
improving	O
scalability	O
of	O
the	O
tool	O
for	O
larger	O
groups	O
,	O
and	O
later	O
PRCS	O
,	O
a	O
simpler	O
CVS-like	O
tool	O
which	O
also	O
uses	O
RCS-like	O
files	O
,	O
but	O
improves	O
upon	O
the	O
delta	O
compression	O
by	O
using	O
Xdelta	B-Application
instead	O
.	O
</s>
<s>
By	O
2006	O
or	O
so	O
,	O
Subversion	B-Application
was	O
considered	O
to	O
be	O
the	O
most	O
popular	O
and	O
widely	O
in	O
use	O
VCS	B-Architecture
tool	O
from	O
this	O
generation	O
and	O
filled	O
important	O
weaknesses	O
of	O
CVS	B-Application
.	O
</s>
<s>
These	O
allow	O
users	O
to	O
maintain	O
independent	O
repositories	O
(	O
or	O
forks	B-Application
)	O
of	O
a	O
project	O
and	O
communicate	O
revisions	O
via	O
changesets	B-Protocol
.	O
</s>
<s>
are	O
some	O
examples	O
of	O
third	O
generation	O
version	B-Architecture
control	I-Architecture
systems	I-Architecture
.	O
</s>
