These adjustable clamp on trammel heads are the perfect tool to measure, mark or scribe circles or arcs outside the range of those that could be produced with a standard compass. Unlimited arcs can be made using your custom-length shop-made beam. These heads will clamp onto any wood beam from 3/8 inch to 1-1/4 inch wide. Set comes with two trammel heads, one with an integrated pencil holder that will accept standard wooden pencils.Comes with an extra set of points. Heads are made from tough cast iron. The points are ground sharp and hardened to allow scribing on steel, sheet metal or aluminum. Overall lenght is 4 inches.