Because definitions of geometric shapes vary widely across textbooks used in the United States, we decided to focus on definitions that are used in some of the more common textbooks, including the one that Nickell used in her classes. Shapedoku puzzles can be adapted to align with any set of definitions that students are learning and using , but the ones shown here focus on classifying triangles on the basis of side lengths and the existence of right angles (topics covered in Nickell’s curriculum) and defining trapezoids as quadrilaterals with exactly one pair of parallel sides (matching the same textbook’s definition). We recognize that this “exclusive definition” is not universally accepted in the United States (Usiskin and Griffin 2007), but it supports Nickell’s students’ conception of a trapezoid. A classification diagram of the thirteen different shapes used in these Shapedoku puzzles is given in figure 5.