  Inflationary consistency equations relate the scalar and tensor
perturbations. We elucidate the infinite hierarchy of consistency equations of
single-field inflation, the first of which is the well-known relation
A_T^2/A_S^2 = -n_T/2 between the amplitudes and the tensor spectral index. We
write a general expression for all consistency equations both to first and
second-order in the slow-roll expansion. We discuss the relation to other
consistency equations that have appeared in the literature, in particular
demonstrating that the approximate consistency equation recently introduced by
Chung and collaborators is equivalent to the second consistency equation of
Lidsey et al. (1997).
