<s>
In	O
a	O
bulletin	B-Application
board	I-Application
system	I-Application
(	O
BBS	O
)	O
,	O
a	O
door	O
is	O
an	O
interface	O
between	O
the	O
BBS	O
software	O
and	O
an	O
external	O
application	O
.	O
</s>
<s>
The	O
term	O
is	O
also	O
used	O
to	O
refer	O
to	O
the	O
external	O
application	O
,	O
a	O
computer	B-Application
program	I-Application
that	O
runs	O
outside	O
of	O
the	O
main	O
bulletin	O
board	O
program	O
.	O
</s>
<s>
Sometimes	O
called	O
external	O
programs	O
,	O
doors	O
are	O
the	O
most	O
common	O
way	O
to	O
add	O
games	O
,	O
utilities	O
,	O
and	O
other	O
extensions	O
to	O
BBSes	B-Application
.	O
</s>
<s>
Because	O
BBSes	B-Application
typically	O
depended	O
on	O
the	O
telephone	O
system	O
,	O
BBSes	B-Application
and	O
door	O
programs	O
tended	O
to	O
be	O
local	O
in	O
nature	O
,	O
unlike	O
modern	O
Internet	O
games	O
and	O
applications	O
.	O
</s>
<s>
Several	O
standards	O
were	O
developed	O
for	O
passing	O
connection	O
and	O
user	O
information	O
to	O
doors	O
;	O
this	O
was	O
usually	O
done	O
with	O
"	O
dropfiles	B-Protocol
"	O
,	O
small	O
binary	O
or	O
text	O
files	O
dropped	O
into	O
known	O
locations	O
in	O
the	O
BBS	O
's	O
file	B-Application
system	I-Application
.	O
</s>
<s>
Most	O
doors	O
were	O
responsible	O
for	O
operating	O
the	O
serial	B-Protocol
port	I-Protocol
or	O
other	O
communications	O
device	O
directly	O
until	O
returning	O
control	O
to	O
the	O
BBS	O
.	O
</s>
<s>
Later	O
development	O
of	O
FOSSIL	B-Protocol
drivers	O
have	O
allowed	O
both	O
BBSes	B-Application
and	O
their	O
doors	O
to	O
communicate	O
without	O
being	O
responsible	O
for	O
direct	O
operation	O
of	O
the	O
communications	O
hardware	O
.	O
</s>
<s>
These	O
games	O
included	O
strategy	O
games	O
such	O
as	O
TradeWars	B-Protocol
2002	I-Protocol
,	O
Food	B-Protocol
Fight	I-Protocol
!,	O
Solar	O
Realms	O
Elite	O
,	O
Space	B-Protocol
Dynasty	I-Protocol
,	O
Usurper	O
,	O
and	O
Barren	O
Realms	O
Elite	O
.	O
</s>
<s>
One	O
such	O
game	O
was	O
Seth	O
Robinson	O
's	O
Legend	B-Protocol
of	I-Protocol
the	I-Protocol
Red	I-Protocol
Dragon	I-Protocol
,	O
and	O
another	O
popular	O
dystopian	O
RPG	O
:	O
Operation	O
:	O
Overkill	O
,	O
another	O
was	O
Mutants	O
!	O
.	O
</s>
<s>
BBSes	B-Application
often	O
published	O
high	O
scores	O
,	O
encouraging	O
players	O
to	O
beat	O
others	O
.	O
</s>
<s>
InterBBS	O
leagues	O
allowed	O
users	O
of	O
different	O
BBSes	B-Application
to	O
compete	O
against	O
each	O
other	O
in	O
the	O
same	O
game	O
.	O
</s>
<s>
While	O
many	O
of	O
the	O
most	O
popular	O
and	O
memorable	O
BBS	B-Protocol
doors	I-Protocol
have	O
been	O
games	O
,	O
numerous	O
doors	O
had	O
non-entertainment	O
applications	O
such	O
as	O
user	O
polls	O
or	O
the	O
time	O
bank	O
,	O
permitting	O
users	O
to	O
time-shift	O
their	O
rationed	O
BBS	O
use	O
.	O
</s>
