While it is true that there are not moons of moons, there are looser
associations between moons, not unlike those in which some minor planets are
associated with Jupiter or Neptune. For example, there is a resonance
between Ganymede, Io, and Europa such that each has twice the month length
of the next in. In each of Ganymede's months the three align with Jupiter
once in the order EJIG and once in the order IJEG. Saturn's moons feature
two overlapping resonances. Dione and Tethys each have a moon with half the
month and dimensions (Enceladus and Mimas respectively) as well as tiny
moons at their leading and trailing Lagrangian points.

"dave black" wrote in message
oups.com...

I've always been puzzled that even 'planet-sized' moons such as Titan
don't have any satellites going round them.

Is there a particular reason?