McFarland, Matt (9 December 2013). "The childhood toys that inspired female engineers and scientists". The Washington Post . Retrieved 10 December 2013. Trapezoid (half hexagon or 3 triangles) (Red) that can be matched with three of the green triangles a b Walter, Marion (December 1996). "letter". Mathematics Teaching (157): 3 . Retrieved 3 May 2020. The EDC Teacher's Guide continues: "Many children start by making abstract designs — both symmetrical and asymmetrical. As play continues these designs may become more and more elegant and complex, or they become simple as the child refines his ideas." All the angles are multiples of 30° (1/12 of a circle): 30° (1×), 60° (2×), 90° (3×), 120° (4×), and 150° (5×).