Also known as Tic-Tac-Toe Ciphers and Masonic Ciphers.
The alphabet is represented by three Noughts and Crosses grids, perhaps using blanks for A to I, dots for J to R (left, centre, right; bottom, middle, top) then crosses for W to Z.
Here’s a message written in the cipher shown above.
Sometimes the last crosses are diagonal and sometimes other symbols are used. You could try creating your own with symbols of your choice.
To complicate your code, you could even randomly input letters into your three grids, making it much harder for others to crack.