  The 2-Opt heuristic is one of the simplest algorithms for finding good
solutions to the metric Traveling Salesman Problem. It is the key ingredient to
the well-known Lin-Kernighan algorithm and often used in practice. So far, only
upper and lower bounds on the approximation ratio of the 2-Opt heuristic for
the metric TSP were known. We prove that for the metric TSP with $n$ cities,
the approximation ratio of the 2-Opt heuristic is $\sqrt{n/2}$ and that this
bound is tight.
