Start with the top row, second square from the right, and move down and to the right in diagonal lines, advancing across the diagram to the left. Letters are written in alphabetical order, in steps of 5 at a time.

Start at the top, and move diagonally down and to the right. Letters advance through the alphabet in steps of 5 then 6. Continue this pattern, starting with the next circle down on the left etc.

Start with the letter in the top left, and move clockwise around the square. The numerical value of the letters follows the sequence of prime numbers, from 2 to 19.