I'm not quite sure if I understand. I mean, I get what it is now, but I'm still a little confused as to its purpose. And is it absolutely nessicary to sew with a scant 1/4" seam if the pattern calls for it? I have enough trouble as it is sewing 1/4" seam (well, maybe not so much...I figured out that its exactly 1/4" inch between my sewing machines needle and the edge of the presser foot)...I can't imagine sewing 1/4" minus a millimeter.