Start on the far left of the diagram and move around it in a clockwise direction. Letters advance through the alphabet in steps of 8 then 9, repeating this pattern all the way around.

Working through the diagram in rows, from top to bottom, the sum of the numerical values of the letters in each row follows the sequence 15, 20, 25, 30, 35

Letters are arranged in each square in alphabetical order, skipping 5 letters at a time. The letter bounded by the central square carries on this sequence, but using the reverse alphabetical value.