Fiskars 8.5-inch x 8.5-inch acrylic square . for dedicated quilters, this is the essential ruler for squaring off finished quilting blocks and centering half square triangles up to 8.5-inch square. the 8.5-inch x 8.5-inch clear acrylic square is made of durable 3mm thick acrylic and has highlighted 1/2-inch seam allowance measurement which is a visual reminder to user which side they are measuring from. the center 45-degree bias line is clearly marked with measurements to aid in trimming blocks. optional non-slip vinyl tabs can be added or removed multiple times without damaging ruler.