Bisecting a Line
Geometric construction for bisecting a line is shown in figure 4-5. To bisect the line AB, use the ends of the line, A and B, as centers; set a compass to a radius greater than one-half the length of AB; and strike arcs intersecting at C and D. A line drawn from C through D bisects AB.
(Source: NAVEDTRA 14069, U.S. Navy)
Lines can also be bisected by using a t square and a triangle.