The Polybius Square
The Greek historian Polybius (c. 200–118 BCE) arranged the alphabet in a 5×5 grid so every letter could be named by two small numbers — its row and its column. He proposed signalling those numbers with two sets of torches held up on a hillside, letting a full sentence cross a valley at the speed of light rather than the speed of a runner. Because the grid holds only 25 cells, I and J share one square.
1
2
3
4
5
1
A
B
C
D
E
2
F
G
H
I/J
K
3
L
M
N
O
P
4
Q
R
S
T
U
5
V
W
X
Y
Z
Encoded pairs
23 11 13 25
H → 23A → 11C → 13K → 25