  The Cartesian product of two cycles (of length m and length n) has a natural
embedding on the torus, such that each face of the embedding is a 4-cycle. The
toroidal grid Qd(m,n,r) is a generalization of this in which there is a shift
by r when traversing the meridian of length m.
  In 2008, Steve Wilson found two interesting infinite families of
(nonbipartite) toroidal grids that are unstable. (By definition, this means
that the canonical bipartite double cover of the grid has more than twice as
many automorphisms as the grid has.) It is easy to see that bipartite grids are
also unstable, because the canonical double cover is disconnected. Furthermore,
there are degenerate cases in which there exist two different vertices that
have the same neighbours. This paper proves Wilson's conjecture that Qd(m,n,r)
is stable for all other values of the parameters.
  In addition, we prove an analogous conjecture of Wilson for the triangular
grids Tr(m,n,r) that are obtained by adding a diagonal to each face of
Qd(m,n,r) (with all of the added diagonals parallel to each other).
