In 1959, in the Netherlands, a short paper appeared entitled"The Child's Thought and Geometry." In it, Pierre van Hiele summarized the collaborative work hat he and his wife, Dieke van Hiele-Geldof, had done describing students' difficulties in learning concepts in geometry. The van Hieles were mathematics teachers l about our middle school level. Dutch students study considerable amount of informal geometry before high school, and the van Hieles observed consistent difficulties from year to year, as their geometry course pogressed. Based on observations of their classes, they stated that learning progresses through stages or levels.