If you look at that AB piece you will notice that it is a parallelogram. To make it you lay your 60 degree ruler on the fabric like any other of the triangles you just cut. Cut or mark the left side and bottom angles. Reverse your ruler and lay it against the cut points of the piece you just partially cut and finish cutting the right side. I recently used this pattern and wanted my table runner longer than the pattern showed. So, I added the required of inches to the top and bottom angle on the right side of the parallelogram and drew another line and cut it. I did the same for the opposite side of the runner. Had just the length I wanted. That is a fun pattern. I am making one right now for my table for Christmas Eve dinner. I have to fussy cut each piece because the pattern is pairs of birds. My fabric looks like swiss cheese-but I think it is worth it. Not sure what to do with the scraps.