Another popular system called a diagrammatic cipher, used by many children in school,
substitutes symbols for letters instead of other letters. This system is, in essence,
the same as the letter substitution system, but it’s easier to remember than
26 randomly picked letters. It uses the tic-tac-toe boards and X`s as shown below.
Although when we used it only one tic-tac-toe board and one X was used and two letters were put in the spaces. When tracing the Dan Brown trail some years ago, I found one of his codes was on a similar system . He just used a tic-tac-toe board and put 3 leters in each space. The dot then came in that space to indicate the letter required in that shape.

The same secret message as above, using the line-shapes that surround each letter
(and including a dot where needed) becomes:
Even though it looks like undecipherable outer-space alien text,
this would take an arm-chair cryptologist only about 10 minutes
or less to figure out. Why? Given enough ciphertext, certain patterns become obvious.
Notice how often the empty four-sided box appears: six times out of a total of 29
characters or about 20% of the time.

This would immediately indicate that the empty box
was almost certainly the symbol for “E,” the most frequently used letter in English.
Other letters can also be determined by their frequency and by their association with
other nearby characters (see “Frequencies”). Almost all substitution ciphers are open
to this kind of analysis.