Early Elementary Math: Angles and Fractions
This piece of equipment is called the Instrument for the Measurement of Angles and is used with the Fraction Circles to let children see how angles are measured in degrees. As your child places a whole circle here, he or she will see how one whole circle forms 360 degrees and then how smaller pieces form different degrees.
DIY tip: If you are making this at home, the measurements must be super precise! You can probably scrounge these used for fairly low prices.
Montessori House math (and more) curriculum for children in early elementary (grades one to three).