This little device helps you to draw a straight line without a ruler. The 'constrained ball' is a drawing aid created by Korean product designer Giha Woo. Once attached to the pen, this device helps to draw straight lines without using a ruler. This writing tool prevents winding lines by helping to move the ball in one direction only. One speciality of this device is, using the installed measuring wheel indicated on the product, users can draw the exact desired line length in both a horizontal and vertical direction, as well as strokes in 45 degree angles.