Da Vinci's Construction of an Ellipse
As vertex of triangle slides along the horizontal axis, vertex slides along the other axis, which does not have to be perpendicular. Vertex then traces out the arc of an ellipse. There can be two solutions for .
Two locators control the position of vertex and the axis angle. You can vary the triangle sides and and the angle at vertex with the slider controls.
This method of drawing an ellipse was found by Leonardo da Vinci.
 D. Wells, The Penguin Dictionary of Curious and Interesting Geometry, London: Penguin, 1991.