Use a compass and a straightedge to construct a line perpendicular to line [math]m[/math] through point [math]B[/math] that is not on the line.
[list=1] [*]Draw line [math] m[/math] with point [math]B[/math] not on the line. [*]Make a large arc that intersects line [math]m[/math]. [*]Make a set of arcs above line [math]m[/math]. [*]Draw the perpendicular line. Do not erase any of your markings. Line [math]n[/math] is perpendicular to line [math]m[/math]. [/list]