Cuisenaire rods were devised in the 1920s by Georges Cuisenaire, a Belgian educator. Similar to how written musical notes make music visible, Cuisenaire rods were designed to make mathematics visible by using wood rods of varying lengths and colour.*
In her first school, and in schools since then, Maria Montessori
used coloured rods in the classroom to teach concepts of both
mathematics and length. Developed independently much later, the Cuisinaire Rods are neverless a kindred spirit to the ones used in the Monressori classrooms.
They are extremely versatile. I have been using them for all my kids, and they become handy to explain basic numeric concepts and not so basic ones...like fractions, factoring, etc. But of course my children also saw the building possibilities... so we also use them for geometry and history.
And I can't wait to use them for grammar, and music, too. (Cuisenaire himself was a math and music teacher)
Here are some of their creations so far: