  In this paper, we introduce the notion of a marginal tube, which is a
hypersurface foliated by marginal surfaces. It generalises the notion of a
marginally trapped tube and several notions of black hole horizons, for example
trapping horizons, isolated horizons, dynamical horizons, etc. We prove that if
every spacelike section of a marginal tube is a marginal surface, then the
marginal tube is null. There is no assumption on the topology of the marginal
tube. To prove it, we study the geometry of spacelike surfaces in a
4-dimensional spacetime with the help of double null coordinate systems. The
result is valid for arbitrary 4-dimensional spacetimes, regardless of any
energy condition.
