It's just a slightly narrow 1/4" seam. I use a 1/4" pressure foot with a metal stop on the outside, so you put fabric up against that and you have the perfect 1/4", move it over so there is a slight space between the fabric and the metal stop and it's a scant 1/4". Mary Ellen Hopkins says that the 1/4" seam is your own PPM (personal private measurement), and as long as you continue using the same 1/4" on the same machine you should be fine. Now I suppose I have made this as clear as mud, but happy quilting anyway.