This 1/2 in. x 15 in. Creative Grids seam marking tool is long enough to mark the diagonal of a 10 in. 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. This is the perfect tool to have on hand when making half-square triangles! This seam marking tool slides easily over the fabric until pressure is applied. Then, the exclusive gripper holds the fabric in place while marking and cutting, eliminating slipping and miss-cuts!