A natural satellite in orbit near the edge of a planetary ring, whose gravitational force on the ring particles strongly controls the distribution of material within the ring, creating ringlets and density waves within the ring and sharp edges at ring boundaries. Examples include Saturn's Prometheus and Pandora, which shepherd the narrow outer F ring and the Uranus satellites Cordelia and Ophelia and the epsilon ring. The faster-moving inside satellite accelerates the inner ring particles as it passes them, causing them to spiral out to larger orbits. At the same time the slower-moving outer satellite decelerates the outer ring particles as they pass by, causing them to spiral inward. The result is a narrow, well-defined ring.