The main role: Before Chartwork for drawing lines bearing line or course, the degree or amount of bearing line degree course line. Two hands each holding a ruler of David, alternately moved in parallel. Usage: When the move operation, the appropriate force to suppress static hand -foot, one hand pushing moving feet, two feet alternately moved in parallel until the required position. Alternately moving, to prevent the static foot displacement., Weight: 2.5794084654 Pounds, Manufacturer: Prterver