This animation is a merging of the Ellipse Device and the Ellipse Foci animations, illustrating that the ellipse device generates a perfect ellipse.

Actually, the Ellipse Device is easier to use when constructing a particular sized ellipse than is the Ellipse Foci method. You always need to know the semi major and semi minor axis for a given ellipse for either construction.

To use the Ellipse Device, the tracing point is set to the dimension of the semi major axis, as measured from the red shuttle. Then the blue shuttle is set to the dimension of the semi minor axis, as measured from the tracing point. Now, draw the ellipse. Easy.

To use the Ellipse Foci method, the Pin and String method, the foci have to be found using the following formula: d = sqrt(a² - b²). The foci, F1 and F2, are located on either side of the center of the ellipse, the distance "d", along the semi major axis. "a" and "b" are the semi major and semi minor axis dimensions respectively. A third point has to be found, the first marking point, so that the string can be tied and used to trace the ellipse. See Ellipse Foci.