The Magnetic Seam Guide attaches to the metal throat plate on a sewing machine. Ideal for ensuring uniform seam width and as a guide to straight stitching. Anti skid design prevents magnet from sliding around while the lightweight, rust-free construction is a step up compared to a traditional metal body. Magnetic Seam Guide - Yellow Measuring Tape, by Sew Tasty. Measures about 2-3/4" x 3/4". Not recommended for use on computerized sewing machines, use ONLY on the sewing machine's needle throat plate.