The Blue Triangles is a Montessori material composed of 12 right-angled blue triangles.
This material familiarizes the child with the ever-recurring right angle, and with the symmetrical shapes made of triangles.
You can create this material together with children as an art project. When you work and play with this material, also help your child in becoming aware of right angles in the room. The child can use one of the triangles to determine if an angle/corner has 90 degrees.
Here you can see the original Montessori material on a picture from the Nienhuis catalog.
I found these easy to follow DIY instructions for the Blue Triangles at montessoriforeveryone.com. Included are some basic shapes that can be formed by the triangles. You can download here:
You can get creative and find many more shapes made of the triangles.
The more famous Chinese game, the Tangram is a similar material. It contains various triangles, a square, and a parallelogram.
These materials help children develop spacial thinking, and lay the foundation for geometry on paper.
Just like the Froebel materials, they can also be used to inspire the child to tell stories and create landscapes and pretend worlds.