<s>
AppleSingle	B-Application
Format	I-Application
and	O
AppleDouble	B-Application
Format	I-Application
are	O
file	O
formats	O
developed	O
by	O
Apple	O
Computer	O
to	O
store	O
Mac	B-Application
OS	I-Application
"	B-Operating_System
dual-forked	I-Operating_System
"	I-Operating_System
files	I-Operating_System
on	O
the	O
Unix	B-Application
filesystem	O
being	O
used	O
in	O
A/UX	B-Operating_System
,	O
the	O
Macintosh	O
platform	O
's	O
first	O
Unix-like	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
AppleSingle	B-Application
combined	O
both	O
file	O
forks	O
and	O
the	O
related	O
Finder	B-Application
meta-file	O
information	O
into	O
a	O
single	O
file	O
,	O
whereas	O
AppleDouble	B-Application
stored	O
them	O
as	O
two	O
separate	O
files	O
.	O
</s>
<s>
Support	O
for	O
the	O
formats	O
was	O
later	O
added	O
to	O
Unix	B-Application
software	O
such	O
as	O
NFS	B-Protocol
and	O
MAE	B-Application
,	O
but	O
they	O
saw	O
little	O
use	O
outside	O
this	O
small	O
market	O
.	O
</s>
<s>
AppleSingle	B-Application
is	O
similar	O
in	O
concept	O
to	O
the	O
more	O
popular	O
MacBinary	B-General_Concept
format	O
,	O
in	O
that	O
the	O
resource	O
and	O
data	B-Operating_System
forks	I-Operating_System
are	O
combined	O
with	O
a	O
header	O
containing	O
the	O
Finder	B-Application
information	O
.	O
</s>
<s>
In	O
fact	O
,	O
the	O
format	O
is	O
so	O
similar	O
,	O
it	O
seemed	O
there	O
was	O
no	O
reason	O
why	O
Apple	O
did	O
not	O
simply	O
use	O
MacBinary	B-General_Concept
instead	O
,	O
which	O
by	O
that	O
point	O
was	O
widely	O
known	O
and	O
used	O
.	O
</s>
<s>
The	O
format	O
was	O
later	O
assigned	O
the	O
MIME	B-Application
type	I-Application
.	O
</s>
<s>
AppleDouble	B-Application
leaves	O
the	O
data	B-Operating_System
fork	I-Operating_System
in	O
its	O
original	O
format	O
,	O
allowing	O
it	O
to	O
be	O
edited	O
by	O
normal	O
Unix	B-Application
utilities	O
.	O
</s>
<s>
The	O
resource	B-Operating_System
fork	I-Operating_System
and	O
Finder	B-Application
information	O
,	O
both	O
proprietary	O
and	O
lacking	O
editors	O
under	O
Unix	B-Application
,	O
were	O
combined	O
into	O
a	O
second	O
file	O
.	O
</s>
<s>
A	O
MIME	B-Application
type	I-Application
was	O
also	O
assigned	O
to	O
AppleDouble	B-Application
,	O
.	O
</s>
<s>
For	O
sending	O
to	O
an	O
AppleDouble	B-Application
un-aware	O
system	O
,	O
the	O
file	O
was	O
generally	O
encoded	O
using	O
Base64	B-Protocol
,	O
as	O
opposed	O
to	O
being	O
converted	O
to	O
AppleSingle	B-Application
.	O
</s>
<s>
Before	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
,	O
AppleSingle	B-Application
and	O
Double	O
had	O
little	O
presence	O
in	O
the	O
Mac	O
market	O
,	O
due	O
largely	O
to	O
the	O
small	O
market	O
share	O
of	O
A/UX	B-Operating_System
.	O
</s>
<s>
Nevertheless	O
,	O
they	O
did	O
force	O
various	O
file	B-General_Concept
compression	I-General_Concept
vendors	O
to	O
add	O
support	O
for	O
the	O
formats	O
,	O
and	O
confuse	O
future	O
MacBinary	B-General_Concept
versions	O
.	O
</s>
<s>
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
revived	O
the	O
use	O
of	O
AppleDouble	B-Application
;	O
on	O
file	O
systems	O
such	O
as	O
NFS	B-Protocol
and	O
WebDAV	B-Protocol
that	O
do	O
not	O
natively	O
support	O
resource	B-Operating_System
forks	I-Operating_System
,	O
Finder	B-Application
information	O
,	O
or	O
extended	O
attributes	O
,	O
that	O
information	O
is	O
stored	O
in	O
AppleDouble	B-Application
format	I-Application
,	O
with	O
the	O
second	O
file	O
having	O
a	O
name	O
generated	O
by	O
prepending	O
"	O
._	O
"	O
to	O
the	O
name	O
of	O
the	O
first	O
file	O
(	O
thus	O
,	O
this	O
information	O
acts	O
as	O
a	O
hidden	B-Application
file	I-Application
when	O
viewed	O
from	O
a	O
non-Apple	O
Unix-based	O
operating	B-General_Concept
system	I-General_Concept
)	O
.	O
</s>
<s>
Metadata	O
separation	O
is	O
also	O
done	O
in	O
the	O
OS	B-Operating_System
X	I-Operating_System
10.3	O
+	O
Finder	B-Application
Zip	B-General_Concept
compression	I-General_Concept
and	O
the	O
command	O
line	O
utility	O
,	O
with	O
a	O
copy	O
of	O
the	O
AppleDouble	B-Application
metadata	O
stored	O
in	O
a	O
directory	O
.	O
</s>
<s>
Unwanted	O
"	O
._	O
"	O
files	O
can	O
be	O
removed	O
using	O
dot_clean	O
-m	O
on	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
.	O
</s>
<s>
Doing	O
so	O
also	O
merges	O
AppleDouble	B-Application
metadata	O
with	O
the	O
corresponding	O
files	O
.	O
</s>
<s>
AppleDouble	B-Application
files	O
can	O
be	O
manually	O
created	O
through	O
creative	O
abuse	O
of	O
(	O
which	O
is	O
AppleDouble-aware	O
)	O
and	O
(	O
which	O
is	O
not	O
)	O
.	O
</s>
<s>
On	O
other	O
systems	O
,	O
the	O
command	O
and	O
a	O
Perl	O
script	O
called	O
can	O
be	O
used	O
to	O
view	O
AppleDouble	B-Application
data	O
.	O
</s>
<s>
Both	O
are	O
part	O
of	O
Netatalk	B-Language
.	O
</s>
<s>
The	O
macOS	B-Operating_System
system	O
provides	O
a	O
set	O
of	O
library	O
functions	O
that	O
allows	O
for	O
packing	O
and	O
unpacking	O
AppleSingle	B-Application
and	O
AppleDouble	B-Application
files	O
in	O
C	O
.	O
</s>
