Boldly marked for great readability these rulers are made of a clear acrylic. Strip Tube Ruler- You'll discover how to make accurate half square triangles quickly! This ruler makes half square triangles a breeze by combining brilliant engineering with "the tube technique." It is perfect to use with strips! Includes directions to make the favorite block from 2½" strips: The Daniela Block. With this great gadget, you can easily make accurate half square triangles from 1" to 9½"!