Create the Second Arc to Find the Vertex

What happens here

Now, shift the pivot point of the string. Place the same end of the string that was at the starting point onto the mark you just made on the straight line (the mark to the right). Hold the Sharpie marker at the other end of the string, keeping it taut. Draw a second arc that intersects the first arc you drew. The intersection point of these two arcs will be the apex of your 60-degree angle, forming an equilateral triangle.

Goal: Locate the vertex of the 60-degree angle by intersecting arcs.