In addition to the answer Kitsie gave you, I am using a wooden dowell about the size of a closet pole cut in half lengthwise to put under the seam, keeps from disturbing the surrounding seams. For a short seam in a block, a length of dowell about 8" works. For a quilt top all put together, a 4' length is adequate. Some people enclose the dowell in a fabric sleeve and close the ends, but I use the wood bare and it has not discolored light fabric. I learned about the dowell from Mrs. Darkow of Washington Stars Quilt Guild.