File talk:Bisection construction.gif

Note that this construction uses non-collapsing compass semantics, so in the purest sense of the compass and straightedge, it wouldn't qualify (at least not without additional intermediate steps). However, it's just as simple to bisect an angle even with a collapsing compass; for the second and third steps, start the compass drawing tip at the vertex whose angle is to be bisected. Then the lower intersection of the circles is coincident with the vertex, so you're just using the straightedge to draw from the upper intersection to the vertex directly. —Klparrot (對話) 18:47, 26 March 2015 (UTC)