  The shock system that produces the afterglow to GRBs consists of a forward-
and a reverse-shock. For short GRBs, observational evidence for a reverse-shock
has been sparse, however, the afterglow to GRB 160821B requires a reverse-shock
at early times to explain the radio observations. GRB 160821B is additionally
accompanied by the best-sampled macronova without a gravitational-wave
detection, and an interesting late-time X-ray afterglow behaviour indicative of
a refreshed-shock. The presence of an observed reverse-shock in an on-axis
short GRB means that the reverse-shock should be considered as a potential
counterpart to gravitational-wave detected mergers. As a gravitational-wave
counterpart, the afterglow to an off-axis GRB jet can reveal the jet structure
-- a reverse-shock will exist in these structured jet systems and the signature
of these reverse-shocks, if observed, can indicate the degree of magnetisation
in the outflow. Here we show the case of GRB 160821B, and how a reverse-shock
will appear for an off-axis observer to a structured jet.
