Large Number Rods: Decomposition

In Montessori Math, the term ‘decomposition’ simply means subtraction. This activity introduces the child to the (minus -) sign.

The child is asked to lay out the floor mat and to bring the number rods carefully, one by one to the floor mat, holding the rods with one hand at the top and the other hand supporting it at the bottom. The directress begins by asking him to build a stair from the shortest rod to the longest rod like before.

Once the stair has been built correctly, the directress brings the number 9-rod and the number 1- rod to the front of the mat and places them down next to each other. The child is encouraged to count the rods, he should count (10). The directress explains to him that we need to take one away, she proceeds to take the number 1-rod away and asks him to count what is left over, he should then say ‘9’. The directress reinforces that we had 10 and we took one away which makes 9. She replaces the rod and moves on to the next operation.

The directress then brings the number 8-rod and the number 2-rod forward, and asks the child to count the rods, he should count (10). She then removes the number 2-rod and asks him to count how many there are now, he should say ‘8’. The directress reinforces that we had 10 and took two away and we are now left with 8. She replaces the rods and moves on to the next operation.

If the child has fully grasped the concept then the directress moves on to the written form. This is done by laying the 7-rod and 3-rod next to each other on the mat. She asks him to count the rods like before, he should count (10). The directress then removes the number 3-rod and asks him to count how much is left. She then brings the whiteboard forward and says to the child ‘that we started with 10 (she writes down 10), we took three away (she writes -3) and that equaled 7 (she writes down =7)

The directress explains to the child what the - sign and the = sign mean. She tells him that the (- sign means to take away and the = sign is how much it makes, and the answer is always written after the = sign.

They replace the rods and move onto the next operation in the same manner. The directress leaves out 10-5 as it is too abstract for him. Once they have completed the sums, she shows the child how to erase the sums on the whiteboard with a duster and assists him in packing the work away.

What the child learns by doing this activity

This activity is an introduction to the - sign, it shows the child how operations are written, as well as it gives the child practice in counting and subtraction.

Looking for more Montessori activities?

Here is a list of all the Montessori Maths activities including Early Maths, Introduction to the Decimal System, Seguin Boards, The Hundred Board, The Short Bead Stair, Bead Chains, The Snake Game, Group Operations, The Large Number Rods, Recording with the Small Number Rods, The Short Bead Stair, The Strip Boards, The Boards, The Stamp Game, The Abacus, The Dot Game, and Fractions. Just click on the page you want to learn about to go there.