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you can use it to draw strait lines with unlimited length,also circle whose radius is between 0cm and 20cm,and arcs with fixed degrees as well.It was quit convinent in both engeneer drawing and daily study.This multifunctionl ruler consists of a straightedge and a disc in the middle of it.There are marks of o-360 degree on the disc,and two round holes in the straightedge.While drawing lines with unlimited length,you can move straightedge and disc alterrately to keep drawing.As drawing circles ,figure out the radius required,fix the disc,put penpoint in the round hole of straightedge and pull it,so that you get a required circle.When draw arcs