  We introduce a notion of a sub-Moebius structure and find necessary and
sufficient conditions under which a sub-Moebius structure is a Moebius
structure. We show that on the boundary at infinity of every Gromov hyperbolic
space Y there is a canonical sub-Moebius structure which is invariant under
isometries of Y such that the sub-Moebius topology on the boundary coincides
with the standard one.
