The maths rods are designed so the single unit piece (1) fits 10 times into the number 10 rod, this means if you place the 10 rod and match it up to the 9 rod you can see that there is room for the single unit piece. Therefore 10 - 9 = 1. This process can be followed for all pieces so 2 x 5 pieces will fit into the 10 piece perfectly. This visual representation helps children to see how maths work.