The boundary between the inner and outer parts of Saturn's B ring is located at the theoretical limit of stability of dust grains with large negative charge to mass ratio. A grain inside of this stability limit will move along (pseudo) magnetic field lines and strike Saturn if given a slight velocity component normal to the ring plane. Outside of this marginal stability radius, a perturbed grain merely oscillates back and forth through the ring plane. The theoretical location of the marginal stability radius is at 1.625 RS. Observations by Pioneer 11 and Voyager 2 in the infrared see the boundary as a prominent change in ring brightness at this radius (within the spatial resolution). The occultation of Δ-Scorpii by the rings in the ultraviolet seen by Voyager 2 shows about a factor of 2 change in optical depth beginning very close to this radius. This close agreement is conceivably a numerical coincidence. Whether this is the case, or whether the instability is actually the physical reason for the boundary's existence, requires further study of the implications.