If you put the 9cm and 2cm rods together end-on-end, the new length is 11cm. Now if you repeat this with the 7cm and 5cm rods that new length is 12cm, so the difference between the two is now 1 cm. It does not seem very practical to walk around measuring items using the rods like this, so you could then make a small rod or use a strip of paper equal to the difference (1cm) as a standard measuring tool.