Indeed it can be signalled in many simple ways (flashing lamps, blasts of sound, drums, smoke signals) and is much easier to learn than more sophisticated codes like the Morse code.
Madison Square Garden | Times Square | Tiananmen Square protests of 1989 | Trafalgar Square | Square | Leicester Square | Tiananmen Square | Red Square | A Nightingale Sang in Berkeley Square (song) | Polybius | A Nightingale Sang in Berkeley Square | Square Enix | Soho Square | Rittenhouse Square | Union Square | Harvard Square | Tompkins Square Park | Thompson Square | Root mean square | Washington Square Park | Syntagma Square | Square (company) | Russell Square | Merrion Square | Independence Square | Hanover Square, London | Fitzroy Square | Cathedral Square | Taksim Square | square |
The cipher system that Uesugi used is basically a simple substitution usually known as a Polybius square or “checkerboard.”