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.  colour:Transparent color  Material:plastic  size:450mm  Package Contents:  1*Chart parallel ruler  Only the above package content, other products are not included.  Note: Light shooting 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.