In the 1880s the Russian revolutionary movement — the Nihilists — ran a war of pamphlets and letters against the Tsar's censorship. Prison correspondence was read and rewritten by guards, so prisoners needed a cipher simple enough to remember and innocent enough to survive a cursory glance. Their answer was the Polybius square plus a short numeric key: the letters became harmless-looking numbers, and the key pushed the numbers around so the plain grid alone decoded nothing.
The scheme spread through the revolutionary underground and into the textbooks as the Nihilist cipher. Its mechanics are a fork of an idea that outlived it: coordinates plus arithmetic. The prison variant that strips the key away again — pure coordinates, tapped on a wall — is the tap code, known to every prisoner of war since. And the addition itself grew up: combine it with a bigger square, several layered numeric keys and a final transposition, and you have the VIC cipher of the 1950s, the last great pencil-and-paper system ever fielded.
The Nihilist cipher sits exactly between the substitution family and the periodic-key family: it is Vigenère's key rhythm applied to numbers instead of letters. Learning it teaches two attacks at once — period finding on a stream, and the clipping constraint that pins each key number — and it is the natural bridge from the Polybius square to ADFGVX, where the same coordinates are fractionated and transposed.