to draw lines in a direction leading to a point it is necessary to locate the ruling edge of a common ruler or straight - edge on the fixed point each time such a thin line is drawn , and in the case of thick lines care has to be taken to put the ruling edge away from the fixed point by a distance equal to one - half the thickness of the line to be drawn . when the fixed point is a comparatively long distance away necessitating the manipulation of a long and hence cumbersome straight - edge , the process becomes tedious and laborious and tends to lack in accuracy . the invented ruler with its telescopic and rotating action combined with its aligning device is intended to facilitate the accurate and quick drawing of such lines .