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. size:450mm Material:plastic colour:White Package Contents: 3 * Chart parallel ruler Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.