IV. Bisecting a line
One of the most basic things in layout is bisecting a line. This simply is a way
to find the center of a line, or anything else for that matter.
On line A-B, set the compass to any distance greater than half the length of the
line. Then using both A and B as centers, swing intersecting arcs above and
below the line.
A line drawn through the intersections will cut line A-B exactly in half.