The treadle I learnt on had zigzag, I remember using it to finish seam edges and to make buttonholes. I still remember the method for making buttonholes. Mark the ends of the hole with pins. Set the zigzag to maximum width, near zero length. Do about 10 stitches. Adjust zigzag to about 3 mm width, sew down one side of buttonhole, do the other end of the buttonhole, turn the fabric then do the second side. Finish the thread with a reverse knot , then cut. Very very carefully cut the buttonhole slit between the two sides of sewing.