They have smooth edges and are crack resistant. They are also made of clear acrylic, so you can see exactly what you are cutting. Measuring with the multiple-size 60 degree ruler is easy because the size of the triangle is measured by the length of a finished side. Cut 3-inch through 12-inch equilateral triangles for patterns like Spider Web, Sugar Loaf Mountain, Thousand Pyramids and more. The ruler has markings every half inch from 3-inch to 12-inch, with quarter inch seam allowance.