Used to measure the distance between points that are too great to be reached with dividers, and for laying out circles. The heads are die cast with black wrinkle finish and have hardened, forged steel divider points. The points screw into the heads. To use with shop-made wooden beam of any length, 3/8" (9.52mm) thick x 3/4" (1.90cm) wide. The adjustable feature is excellent.