Bloc Loc's Half Rectangle Triangle Ruler set includes 2 grooved rulers in a 2:1 Ratio. The 2:1 ratio is the industry standard for quilt patterns. Each ruler measures 2.5" x 4.5" and can be used for any size HRT unit smaller than the rulers. These rulers can be used for smaller projects, and work for blocks up to 12 inches. WORKS WITH THE BLOCK MAKER AND BLOCK MAKER II BOOKS.