Creative Grids 9" Seam Guide Tool. This 1/2" x 9" seam marking tool is long enough to mark the diagonal of a 6.5" 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 15" length long enough to mark the diagonal 10" square.