Two plain squares sit top left and bottom right. Two key squares sit top right and bottom left. Take your letters in pairs: find the first in the top left square and the second in the bottom right, then read off the two corners that complete the rectangle, from the key squares.
Like Playfair it works on pairs, so counting single letters is useless. Unlike Playfair it never needs an X pushed between double letters, which is why some people prefer it.
Playfair without the awkward bit
Felix Delastelle spent his life inventing ciphers that fixed the annoyances in other people’s. The four-square does what Playfair does without ever needing an X pushed between double letters, and the bifid on this site is his as well. He published his book the year he died, and the ciphers outlived him by a century.
Print this page and you get a worksheet: how the cipher works, where it came from, and the puzzles to crack, with the machine and the menus left off.
Print this page as a worksheet Its sheet in the pack Try the challengesLonger, harder and written for grown-ups. Worth a look when this page runs out.