Each resonance consists of a chain of n islands and each island has a structure similar to the pendulum. Perturbation theory implies that the width of the m/n resonance grows as K n/2 for small K. At the center of the island, and at the cusp of the separatrix, are periodic orbits with frequency m/n. Typically there appear to be only two such periodic orbits. Orbits trapped in an island move successively from one island to another, following the periodic orbit (they skip m-1 islands each step). Thus there is an entire region of phase space that has frequency m/n.