The Box of Blue Triangles is a geometry material containing a set of equilateral, right-angled, and obtuse blue triangles that children combine to construct larger geometric figures, directly experiencing how triangles are the basis of all rectilinear forms. It is typically introduced in the primary environment after the child has worked with the Geometric Cabinet and is developing a more analytical understanding of plane figures. Having this material available extends constructive triangle work and gives children a focused, open-ended exploration that supports both geometric reasoning and spatial thinking.