Creative Grids 15" Seam Guide Tool. This 1/2" x 15" seam marking tool is long enough to mark the diagonal of a 10" square. Mark through the slotted center with any mechanical pencil and sew 1/4" on each side of the tool. Or mark the sewing line by drawing on both outside edges. Creative Grids exclusive grip holds the tool in place while marking-the perfect tool when making half-square triangles! This tool also comes in a 9" length long enough to mark the diagonal 6" square.