My mother was very stern about my unstraight seams. I didn't sew much because of it. I would sew and see the crooked seams and just put it away. Well now retired and loving to sew, I started taking classes at my LQS. Same problem not one of my many blocks was 12 1/2 inches when finished. One of the instructors finally taught me and many others (seems this is not an uncommon problem) the way her mother taught her. She brought in a cool ruler, it has a hole in it where the scant 1/4 inch would sew. The edge of the ruler would be where you would line up your fabric for that seam. We then took moleskin (Dr. Schules foot products, in pharmacy area) cut it into strips, butted one up to ruler - voila, perfect scant straight 1/4" seams. Luv it and am so proud of my sewing!!!! Wish my mom were alive to show her. You can find one of these rulers at most sewing stores, the moleskin any Wal-Mart, K-Mart, etc. The moleskin is thicker so it is easier to follow than tape. Try it. My last squares were perfectly 12 1/2.