The number of black dots in each grid increases by 1 each time, starting with the top left grid and working to the right, top row then bottom row.

Adding the three numbers in each square together gives the numerical value of the letter at the centre of each square.

The value at each corner of the diagram equals the difference between the sums of the numerical values of the letters in the boxes adjacent to the corner.

Convert each letter to its numerical value, and read each pair of values as 2 digit numbers.

In each row, the number in the centre equals the difference between the 2 digit values on the left and right.

Start in the top left hand corner and move anti clockwise around the perimeter of the square.

Letters are written in alphabetical order, skipping 1 letter, then 2 letters, then 3 etc.