The Omnigrid 12-1/2? x 12-1/2? ruler features an extra ½? width for seam allowances; it is used to cut, square and trim blocks sized 12-1/2? and under when quilting. In general, Omnigrid square rulers are used for secondary cutting of strips, and to square up finished quilt blocks Use with a rotary cutter or a straight-edge blade The 12-1/2? x 12-1/2? ruler has the following standard Omnigrid features: Diagonal angle line Edge markings for accurate measuring Laser cut edges for smooth, deep cutting Patented double-sight lines that show up on light and dark fabrics Heavy-duty durable acrylic material Left- and right-hand numbering and angle system