The "Ring-Masters" of Saturn

Caption NASA:"Two moons that have profound impacts on the Rings of Saturn, Mimas and Prometheus, are seen here with the F-Ring.

Mimas (approx, 396 Km, or about 246 miles across), the larger and much more distant of the moons, creates the Cassini Division: between the A and B-Rings.
Prometheus (approx. 86 Km, or about 53 miles across), although much smaller than Mimas, is half of a duo responsible for maintaining the narrow FRing".