This is a technique for drawing irregular symmetrical forms. I created this technique while drawing a guitar. First, draw one side of the shape you want to be symmetrical, but is irregular. You can draw just a part of the side, rather than the whole thing. Second, take a blank piece of scrap paper, put it over the line you drew, and trace it. Fourth, lift up the scrap paper. There will be a faint image of the reverse mirror image of the line on your drawing and it will be in the proper place if you positioned it right. Use this faint image to draw the other side of the form.