To divide a board into thirds, choose any number that’s divisible by three, like nine, 12, 18, etc. Lay a square or tape measure diagonally across the board with zero at one end and in this case 18 at the other.
Mark the 1/3 spot (6 in.) and the 2/3 spot (12 in.).
Use those marks to draw the lines dividing up the board.
Divide a board into quarters by choosing a number divisible by four, fifths by choosing a number divisible by five and so on. This trick will work for any board no matter how wide it is.