Polybius, the Greek historian of the second century BC, describes a signalling scheme for sending any message across the sea and hills: two groups of five torches, raised left and right, spell one letter at a time — a number pair for each letter. The square was not built for secrecy but for speed and range: a code both endpoints could work out in the dark, letter by letter, with nothing written down.
The scheme sleeps for two thousand years and then reappears everywhere. Clergy and prisoners tap it out on walls — five-by-five, two knocks for the row, two for the column, and a pause between letters. Russian revolutionaries of the 1880s gave the square a numeric overlay and a cipher of their own (the Nihilist cipher, later in the ladder). The French army in 1918 built its most sophisticated field cipher, ADFGVX, on the same 25 (later 36) cells, then scrambled the coordinates with a transposition.
The square is the bridge between two halves of classical cryptography: it is the simplest substitution that produces a numeric ciphertext, and numbers invite arithmetic — keys added to coordinates, coordinates fractionated, mixed and transposed. Every later stop on this ladder that touches a grid — Polybius square at its centre — starts here.
The Polybius page holds the plain idea: coordinates in, coordinates out, a live grid to watch. Its descendants each get their own page — Nihilist, ADFGVX, and the tap code in the ladder's later stretch.