<s>
In	O
some	O
versions	O
of	O
the	O
Unix	B-Application
operating	I-Application
system	I-Application
,	O
the	O
term	O
universe	O
was	O
used	O
to	O
denote	O
some	O
variant	O
of	O
the	O
working	O
environment	O
.	O
</s>
<s>
During	O
the	O
late	O
1980s	O
,	O
most	O
commercial	O
Unix	B-Application
variants	O
were	O
derived	O
from	O
either	O
System	B-Operating_System
V	I-Operating_System
or	O
BSD	B-Operating_System
.	O
</s>
<s>
Most	O
versions	O
provided	O
both	O
BSD	B-Operating_System
and	O
System	B-Operating_System
V	I-Operating_System
universes	O
and	O
allowed	O
the	O
user	O
to	O
switch	O
between	O
them	O
.	O
</s>
<s>
Each	O
universe	O
,	O
typically	O
implemented	O
by	O
separate	O
directory	O
trees	O
or	O
separate	O
filesystems	O
,	O
usually	O
included	O
different	O
versions	O
of	O
commands	O
,	O
libraries	B-Library
,	O
man	B-Protocol
pages	I-Protocol
,	O
and	O
header	B-Language
files	I-Language
.	O
</s>
<s>
While	O
such	O
a	O
facility	O
offered	O
the	O
ability	O
to	O
develop	O
applications	O
portable	O
across	O
both	O
System	B-Operating_System
V	I-Operating_System
and	O
BSD	B-Operating_System
variants	O
,	O
the	O
requirements	O
in	O
disk	O
space	O
and	O
maintenance	O
(	O
separate	O
configuration	O
files	O
,	O
twice	O
the	O
work	O
in	O
patching	O
systems	O
)	O
gave	O
them	O
a	O
problematic	O
reputation	O
.	O
</s>
<s>
Systems	O
that	O
offered	O
this	O
facility	O
included	O
Harris/Concurrent	O
'	O
s	O
CX/UX	O
,	O
Convex	O
's	O
Convex/OS	O
,	O
Apollo	O
's	O
Domain/OS	B-Application
(	O
version	O
10	O
only	O
)	O
,	O
Pyramid	O
's	O
DC/OSx	B-Operating_System
(	O
dropped	O
in	O
SVR4-based	O
version	O
2	O
)	O
,	O
Concurrent	O
's	O
Masscomp/RTU	O
,	O
MIPS	O
Computer	O
Systems	O
 '	O
RISC/os	B-Operating_System
,	O
Sequent	O
's	O
DYNIX/ptx	B-Operating_System
and	O
Siemens	O
 '	O
SINIX	B-Operating_System
.	O
</s>
<s>
Some	O
versions	O
of	O
System	B-Operating_System
V	I-Operating_System
Release	O
4	O
retain	O
a	O
system	O
similar	O
to	O
Dual	O
Universe	O
concept	O
,	O
with	O
BSD	B-Operating_System
commands	O
(	O
which	O
behave	O
differently	O
from	O
classic	O
System	B-Operating_System
V	I-Operating_System
commands	O
)	O
in	O
,	O
BSD	B-Operating_System
header	B-Language
files	I-Language
in	O
and	O
library	O
files	O
in	O
.	O
</s>
<s>
can	O
also	O
be	O
found	O
in	O
NeXTSTEP	B-Application
and	O
OPENSTEP	B-Operating_System
,	O
as	O
well	O
as	O
Solaris	B-Application
.	O
</s>
