The pigpen cipher (alternately referred to as the masonic cipher, Freemason's cipher, Napoleon cipher, and tic-tac-toe cipher) is a geometric simple substitution cipher, which exchanges letters for symbols which are fragments of a grid1.
You can get the idea by looking at the cipher key below.
With this key, an example enciphered message would look like this:
However, the key above gives just one variant of how the letters can be assigned to the grid. There are other variants. For example, a different variant of the cipher key was used in Assassin's Creed II game - letters were paired, e.g. AB, CD, etc., and the symbol with the dot was substituted for the second letter in the pair. Other variants may include starting the grid with the initial word, e.g. MASON, and then using all remaining letters in order, etc.
That's why I've created more or less universal pigpen cipher decoder. The idea is to let you add another variants of the key, and use them for deciphering. I've added the one used in wikipedia, and the one used in Assassin's Creed II. Any registered user of this site can add another by adding new handbook item following this link. Note that handbook item should include picture of the key for the convenience and the size of the picture should be 240x240.