<s>
The	O
tap	B-Algorithm
code	I-Algorithm
,	O
sometimes	O
called	O
the	O
knock	B-Algorithm
code	I-Algorithm
,	O
is	O
a	O
way	O
to	O
encode	O
text	O
messages	O
on	O
a	O
letter-by-letter	O
basis	O
in	O
a	O
very	O
simple	O
way	O
.	O
</s>
<s>
The	O
tap	B-Algorithm
code	I-Algorithm
has	O
been	O
commonly	O
used	O
by	O
prisoners	O
to	O
communicate	O
with	O
each	O
other	O
.	O
</s>
<s>
The	O
tap	B-Algorithm
code	I-Algorithm
is	O
based	O
on	O
a	O
Polybius	O
square	O
using	O
a	O
5×5	O
grid	O
of	O
letters	O
representing	O
all	O
the	O
letters	O
of	O
the	O
Latin	O
alphabet	O
,	O
except	O
for	O
K	B-Application
,	O
which	O
is	O
represented	O
by	O
C	B-Language
.	O
</s>
<s>
For	O
example	O
,	O
to	O
specify	O
the	O
letter	O
"	O
B	B-Language
"	O
,	O
one	O
taps	O
once	O
,	O
pauses	O
,	O
and	O
then	O
taps	O
twice	O
.	O
</s>
<s>
The	O
letter	O
"	O
X	B-Operating_System
"	O
is	O
used	O
to	O
break	O
up	O
sentences	O
,	O
and	O
"	O
K	B-Application
"	O
for	O
acknowledgements	O
.	O
</s>
<s>
To	O
simulate	O
Morse	O
by	O
tapping	O
therefore	O
requires	O
either	O
two	O
different	O
sounds	O
(	O
pitch	O
,	O
volume	O
)	O
,	O
or	O
very	O
precise	O
timing	O
,	O
so	O
that	O
a	O
dash	O
within	O
a	O
character	O
(	O
e.g.	O
</s>
<s>
the	O
character	O
N	B-Application
,	O
)	O
remains	O
distinguishable	O
from	O
a	O
dot	O
at	O
the	O
end	O
of	O
a	O
character	O
(	O
e.g.	O
</s>
<s>
E-E	O
,	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
person	O
hears	O
four	O
knocks	O
,	O
they	O
can	O
think	O
"	O
A	O
...	O
F	B-Language
...	O
L	O
...	O
Q	B-Algorithm
"	O
.	O
</s>
<s>
If	O
after	O
a	O
pause	O
there	O
are	O
three	O
knocks	O
,	O
they	O
think	O
"	O
Q	B-Algorithm
...	O
R	B-Language
...	O
S	B-Language
"	O
to	O
arrive	O
at	O
the	O
letter	O
S	B-Language
.	O
</s>
<s>
Like	O
the	O
"	O
knock	B-Algorithm
code	I-Algorithm
"	O
,	O
a	O
Cyrillic	O
script	O
version	O
is	O
said	O
to	O
have	O
been	O
used	O
by	O
nihilist	O
prisoners	O
of	O
the	O
Russian	O
czars	O
.	O
</s>
<s>
The	O
knock	B-Algorithm
code	I-Algorithm
is	O
featured	O
in	O
Arthur	O
Koestler	O
's	O
1941	O
work	O
Darkness	O
at	O
Noon	O
.	O
</s>
<s>
Kurt	O
Vonnegut	O
's	O
1952	O
novel	O
Player	O
Piano	O
also	O
includes	O
a	O
conversation	O
between	O
prisoners	O
using	O
a	O
form	O
of	O
tap	B-Algorithm
code	I-Algorithm
.	O
</s>
<s>
The	O
code	O
used	O
in	O
the	O
novel	O
is	O
more	O
primitive	O
and	O
does	O
not	O
make	O
use	O
of	O
the	O
Polybius	O
square	O
(	O
e.g.	O
</s>
<s>
"	O
P	B-Language
"	O
consists	O
of	O
sixteen	O
taps	O
in	O
a	O
row	O
)	O
.	O
</s>
<s>
United	O
States	O
prisoners	O
of	O
war	O
during	O
the	O
Vietnam	O
War	O
are	O
most	O
known	O
for	O
having	O
used	O
the	O
tap	B-Algorithm
code	I-Algorithm
.	O
</s>
<s>
It	O
was	O
introduced	O
in	O
June	O
1965	O
by	O
four	O
POWs	B-Application
held	O
in	O
the	O
Hỏa	O
Lò	O
(	O
"	O
Hanoi	O
Hilton	O
"	O
)	O
prison	O
:	O
Captain	O
Carlyle	O
"	O
Smitty	O
"	O
Harris	O
,	O
Lieutenant	O
Phillip	O
Butler	O
,	O
Lieutenant	O
Robert	O
Peel	O
,	O
and	O
Lieutenant	O
Commander	O
Robert	O
Shumaker	O
.	O
</s>
<s>
Harris	O
had	O
heard	O
of	O
the	O
tap	B-Algorithm
code	I-Algorithm
being	O
used	O
by	O
prisoners	O
in	O
World	O
War	O
II	O
and	O
remembered	O
a	O
United	O
States	O
Air	O
Force	O
instructor	O
who	O
had	O
discussed	O
it	O
as	O
well	O
.	O
</s>
<s>
In	O
Vietnam	O
,	O
the	O
tap	B-Algorithm
code	I-Algorithm
became	O
a	O
very	O
successful	O
way	O
for	O
otherwise	O
isolated	O
prisoners	O
to	O
communicate	O
.	O
</s>
<s>
POWs	B-Application
would	O
use	O
the	O
tap	B-Algorithm
code	I-Algorithm
in	O
order	O
to	O
communicate	O
to	O
each	O
other	O
between	O
cells	O
in	O
a	O
way	O
which	O
the	O
guards	O
would	O
be	O
unable	O
to	O
pick	O
up	O
on	O
.	O
</s>
<s>
It	O
was	O
even	O
used	O
when	O
prisoners	O
were	O
sitting	O
next	O
to	O
each	O
other	O
but	O
not	O
allowed	O
to	O
talk	O
,	O
by	O
tapping	O
on	O
another	O
's	O
thigh	O
.	O
</s>
<s>
By	O
overcoming	O
isolation	O
with	O
the	O
tap	B-Algorithm
code	I-Algorithm
,	O
prisoners	O
were	O
said	O
to	O
be	O
able	O
to	O
maintain	O
a	O
chain	O
of	O
command	O
and	O
keep	O
up	O
morale	O
.	O
</s>
