Arc Complement

Reverses the start and end angles of an arc. In other words, the arc will become the portion of the original circle that was cut.

Select the arc you want to modify.
Click anywhere to create the arc complement.

Tip: When using this tool on arcs that are the result of exploding a polyline, or created as the result of using the Arc Divide option of the Edit Tool, this tool may select more than one arc at a time. If this happens, move the arcs from each other.