Working on from the above example, we are going to add a second geometric shape. This shape resembles a cross, but we use it the same way. We now put some of letters on the noughts and crosses board and some on the cross.
Again we use a mark to indicate which letter in that area we are referring to. This time we use a single mark, as there are only two possible letters per space. In our example having the mark there means use the second letter, but you could reverse it's meaning to mean use the first letter.
Try using what you've learnt to crack our cyphers. (Answers are in the video see here.)