This design of Bob's using 8 half-squares, four red and four green, statrted from a ground form of a square that was diagonally split between red and green colors

After some transformations he had an enclosed pinwheel! Joanne saw in his design groups of butterflies , each made of two triangles with their right angles touching point-to-point.

But one "butterfly" is not like the others. Bob, ever in a playful mood, liked his design with the "flaw" because it added an element of "intrique".