Trammel of Archimedes
The ellipse is drawn automatically from the three blue dots, which specify the centre, major axis and minor axis. The focal points are marked in red.
The blue lines have length equal to half the sum of the major axis and minor axis. On each blue line, the distance between the black dots is equal to half the minor axis. The theorem says that if one of these black dots is on the extension of the minor axis then the other lies on the ellipse.
Similarly, the red lines have length equal to half of the major axis and the distance between the two black dots is half the minor axis. The theorem says that if one black dot lies on the major axis then the other lies on the ellipse.