  We revisit the theory of null shells in general relativity, with a particular
emphasis on null shells placed at horizons of black holes. We study in detail
the considerable freedom that is available in the case that one solders two
metrics together across null hypersurfaces (such as Killing horizons) for which
the induced metric is invariant under translations along the null generators.
In this case the group of soldering transformations turns out to be infinite
dimensional, and these solderings create non-trivial horizon shells containing
both massless matter and impulsive gravitational wave components. We also
rephrase this result in the language of Carrollian symmetry groups. To
illustrate this phenomenon we discuss in detail the example of shells on the
horizon of the Schwarzschild black hole (with equal interior and exterior
mass), uncovering a rich classical structure at the horizon and deriving an
explicit expression for the general horizon shell energy-momentum tensor. In
the special case of BMS-like soldering supertranslations we find a conserved
shell-energy that is strikingly similar to the standard expression for
asymptotic BMS supertranslation charges, suggesting a direct relation between
the physical properties of these horizon shells and the recently proposed BMS
supertranslation hair of a black hole.
