Originally Posted by
platyhiker
One way to get two really good straight edges to join together is to place the pieces of batting so that they are overlapping, and then put your longest ruler on top of the overlapping section and use your rotary cutter to cut along the ruler's edge. You discard the two narrow strips cut from each piece of batting, butt the clean edges together (no overlap), and then stitch them together.
This is the method that I use. But I add a few pieces of tape that is a water soluable stabilizer. I put it across the seam line and zig zag. The tape washes away when the quilt is washed.