Charles Wheatstone — scientist, showman, bridge-builder (the measurement bridge carries his name) — invented the cipher in 1854. But it is named for his friend Lord Playfair, the scientist-politician who promoted it to the British establishment. Playfair demonstrated it at a dinner party to the leadership of the Foreign Office and the Post Office; the reaction was famously dismissive — too difficult, officials decided, for their clerks to master.
The military, less precious about its clerks, adopted what the diplomats refused. The cipher served British forces in the Second Boer War of 1899 as a field cipher for tactical messages, and again through the First World War. Its virtues were practical: no tables to carry beyond a memorised square, no equipment, quick enough for a junior officer with a pencil.
What made Playfair new was not the square — squares of letters were known — but the pairing: encrypting two letters at once. That single change broke the letter-frequency shortcut that had ruled since Al-Kindi, and it took cryptanalysis a new statistical tool (digraph frequencies) to recover the advantage. For a classical cipher, that is a long life: it protected field traffic for some sixty years.
The idea of encrypting grouped units became a design pattern: two-square and four-square ciphers split and doubled the square for more mixing, and Bifid-style ciphers layered fractionation on top. The direct line ends at the machines — rotor systems substituted whole alphabets on the fly — but the insight that context changes the meaning of a letter is Playfair's permanent gift.