A = 1 _ 1
B = 1 _ 2
C = 1 _ 3
D = 1 _ 4
E = 1 _ 5
F = 2 _ 1
G = 2 _ 2
H = 2 _ 3
I = 2 _ 4
J = 2 _ 5
L = 3 _ 1
M = 3 _ 2
N = 3 _ 3
O = 3 _ 4
P = 3 _ 5
Q = 4 _ 1
R = 4 _ 2
S = 4 _ 3
T = 4 _ 4
U = 4 _ 5
V = 5 _ 1
W = 5 _ 2
X = 5 _ 3
Y = 5 _ 4
Z = 5 _ 5

Legend:

_ represents space or pause before tapping the second number.
The numbers represent how many times you tap.

The tap code, sometimes called the knock code, is a way to encode text messages on a letter-by-letter basis in a very simple way. The message is transmitted using a series of tap sounds, hence its name.

The tap code has been commonly used by prisoners to communicate with each other. The method of communicating is usually by tapping either the metal bars, pipes or the walls inside a cell.

It is based on a Polybius square using a 5×5 grid of letters representing all the letters of the Latin alphabet, except for K, which is represented by C.

The listener only needs to discriminate the timing of the taps to isolate letters.

Each letter is communicated by tapping two numbers:

• the first designating the row
• the second designating the column

For example, to specify the letter "B", one taps once, pauses, and then taps twice.

Note : the pause between each number in a pair is smaller than the pause between letters.