Taking the numerical value of each letter, they follow the sequence of Prime Numbers.

Starting with the A on the left hand side, move around the shape in a clockwise spiral towards the centre.

Letters advance through the alphabet in steps of 4, 5 then 6, repeating this pattern all the way around.

Starting with A and C, as you move clockwise, the numerical value of the next letter equals the sum of the preceding two letters.

As you move down, letters appear in alphabetical order, skipping those letters only written with straight lines.

Starting at the top left, and moving to the right, then down one row and to the left etc. in a snakes and ladders pattern, letters advance through the alphabet by 5 steps, then back 2, repeating this pattern for the rest of the diagram.