This practice shows how to draw perpendicular lines.
It was stated in Fractions of the Ruler that two perpendicular lines were created when a compass was used to divide a line into equal segments. If you want to draw a perpendicular from a particular point on a line, the process is the same; however, the approach is a little different.
First: Draw a straight line of any length and mark a point on the line.
Second: Using the same radius setting, place the metal end point of the compass on the marked point and mark the line on both sides of the point.
This makes the original point on the line a center point of the segment.
Third: Set your compass a little longer than the distance from one of the end points of the segment to the center point. Set your compass point on an endpoint of the segment and mark an arc above the center point.
Fourth: Keep the compass setting the same and repeat the action from the other end point.
Fifth: Draw a straight line from the point where the arcs cross to the center point of the line.
These two straight lines are perpendicular to each other.