A two-piece tool, forming a beam compass, used to layout circles or arcs. Often a length of 1x stock is used as the beam, which becomes the radius. For longer distances, 1x steel is preferred. One piece has a point which becomes the center, the other has a pencil which marks the circumference.
http://www.micromark.com/Clamp-On-Trammel-Points-Set-Of-2,8389.html
http://www.micromark.com/Clamp-On-Trammel-Points-Set-Of-2,8389.html