The Add-a-Quarter with its specially designed lip automatically adds a 1/4" seam allowance and provides a straight edge for your rotary cutter. When sewing spirals, it's essential for doing a "Next Step Trim". An absolute "must" tool for paper-piecing, this is one of my "can't live without it" favorites! (When sewing pieces of fabric that finish to less than 1/2" high, use the Add-an-Eighth Ruler.)