There is a sequence of approximate ratios, 3:4:5:6, between the periods of Styx, Nix, Kerberos, and Hydra with that of Charon; the ratios become closer to being exact the further out the moons are.
Terdapat pula urutan rasio terdekat 3:4:5:6 antara periode Styx, Nix, Kerberos, dan Hydra dengan periode Charon; nilai tersebut mendekati pasti bila urutannya dibalik.