The black lines on each side of the red seam line replicate a perfect 1/4" seam, so getting a perfect 1/4", lining up fabric, or even making half square triangles is easy. Let the tape act as a guide for sewing diagonal seams without marking lines. The tape is strong, adheres well to every surface, but removes cleanly when not left for long periods of time. 'Diagonal Seam Tape', 10yd roll, by Cluck Cluck Sew.