Serrated wheel 1 rotates about axis A. Lever 2, carrying ruling pen 3, turns about fixed axis B and its end a engages serrated wheel 1. As wheel 1 is run along the bevelled edge of the T-square, it rotates and its teeth turn lever 2 and the rigidly attached pen 3 clockwise. When end a of lever 2 drops into a tooth space, lever 2 is turned counterclockwise by spring 4, raising pen 3. Thus a dot or dash line is drawn on the paper.
$533$SL,FD$