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The Smitty Harris Tap Code is an alternative to morse code that is used in situations where it is impossible to vary the duration of the signals. The ordinary morse code consists of two elements, the dit (short signal) and the dah (long signal), with different lengths, but it is impossible to send a dah by knocking on a wall, or striking a bell. Therefore, Vietnam prisoners of war developed a code with only dots, separated by different amounts of spacing.

Unlike morse code, there is no correspondence between the frequency and length of the codes. Each letter is transmitted by 2 series of impulses, each series ranging from 1 to 5 impulses. Thus, the Smitty Harris alphabet has a total of 25 letters. C and K have the same code, to make it possible to arrange the English alphabet in a neat 5x5 table. The codes are as follows:

A
. .
B
. ..
C/K
. ...
D
. ....
E
. .....
F
.. .
G
.. ..
H
.. ...
I
.. ....
J
.. .....
L
... .
M
... ..
N
... ...
O
... ....
P
... .....
Q
.... .
R
.... ..
S
.... ...
T
.... ....
U
.... .....
V
..... .
W
..... ..
X
..... ...
Y
..... ....
Z
..... .....