Bisecting angles has to be one of the most common activities involved in both drawing fortifications and studying the plans of old fortifications. This construction is most often used to determine the location and direction of the capitals of salient angles.

Given an angle ABC bisect the angle into two smaller equal angles.

Using the vertex (Point B) as center describe an arc of a circle of any radius that cuts both leg BC and leg AB.

Marking the points where the arc cuts the legs as Point D and Point E and using Point D as center describe an arc of a circle that has a radius equal to the distance between Points D and E.

Using Point E as center, describe a second arc that has a radius equal to the distance between Points D and E. Mark the point where this arc intersects the arc centered on Point D as Point F.

Using a straightedge produce a line that passes from the vertex of the angle (Point B) through Point F.

~ An Instructive Animation Wherein the Foregoing Construction is Visually Explained ~