We introduce the notions of
Heegaard splittings and thin multiple Heegaard splittings of 1-submanifolds in
compact orientable 3-manifolds, which are generalizations of those of bridge
decompositions and thin positions. We show that either a thin multiple Heegaard
splitting of 1-submanifold T is also a Heegaard splitting with minimal complexity or
the exterior of T contains an essential surface with meridional boundary other than
the boundary parallel annulus.