Place the Diagonal Seam Tape in front of the feed dogs of the sewing machine, with the red stitching line in line with the needle. The black lines represent a perfect 1/4" seam allowance, which helps guide fabric, make HST's without marking lines, and is a great help for beginners learning to sew and keeping long strips of fabric straight. Diagonal Seam Tape is washi tape, so it's strong, thin, and removes cleanly. Remove it after each project to prevent any residue on the machine. Each roll is 10 yards.