Tic-Tac-Toe code

Tic-Tac-Toe code is the code used by the EPF (and formerly the PSA) to write secret messages. It is so named because the symbols it uses are taken from different parts of a Tic-Tac-Toe grid.

Trivia

 * It is a variation on the real-life pigpen cipher.
 * The signature of the Director of the PSA is the symbol that would come after Z in Tic-Tac-Toe code.