Piecing Strange Angles
Sometimes you have to join two strangely cut pieces and if you don’t line them up correctly, the end result will be off.
First put one of the pieces on your mat so that the seam side is on a line (here it is along the 10" line), and the angled end crosses at the 1/4″ mark (here it is at the 10 1/4" mark).
Then put the piece to be sewn on top RST so that the ends cross at the 1/4″ point at shown. Your 1/4″ seam line should pass off the fabric at this point when you sew.