This 1/2 x15 inch seam marking tool is long enough to mark the diagonal of a 10 inch square. Mark through the slotted center with any mechanical pencil and sew 1/4 inch on each side of the tool. Or mark the sewing line by drawing on both outside edges. Our exclusive grip holds the tool in place while marking - the perfect tool when making half-square triangles! This tool also comes in a 9 inch length - long enough to mark the diagonal of a 6 inch square (Item CGRSMN9).