Most people use a square to make things straight or square, but it doesn't have to be that way. It can be used to make graceful curves as well. Here is how to draw an ellipse with a framing square. Use it to lay out a gentle arch over a window or doorway.
Take a straight piece of wood and put two brads into it. The first goes at a distance equal to the height of the ellipse (shown as distance "A" in red). The second is at half the width of the ellipse (shown as distance "B" in Blue. Place the square with the inside corner at the center of where you want the ellipse. Tape or hotglue a pencil or piece of pencil lead at the end of you stick. Run the brads along the inside edges of the square. Flip the square and then do the other side.