The Pigpen Cipher

Also called the Freemason's cipher, this scheme lays the alphabet across four pens: two tic-tac-toe grids and two X grids. A letter is written as the walls of the cell that holds it, with a dot added for the second grid of each pair. Freemasons and Rosicrucians used it from the 18th century onward to keep lodge records, ritual notes, and even gravestone inscriptions private — it hides meaning, not the fact that something is hidden.