The trammel of Archimedes is a device that circumscribes an ellipse by allowing two pivot points fastened to a rod to slide along two grooves perpendicular to each other.

The above slimulation lets you play with a trammel:

• Move the green dot around to swing the trammel arm around the ellipse.
• The red dot shows the position of a marking device.
• You can click the blue arrows to change the distance between the blue pivot points.
• The green arrows change the size of the ellipse.