  We establish a classical analog of the Nambu-Goldstone theorem for
spontaneous breaking of spacetime symmetries. It provides a counting rule for
independent Nambu-Goldstone fields and states which of them are gapped. We
demonstrate that only those symmetry group generators give rise to independent
Nambu-Goldstone fields that act non-trivially on a vacuum at the origin of
coordinates. Other generators give rise to auxiliary fields that must be
excluded from a theory by the means of inverse Higgs constraints. The physical
meaning of the inverse Higgs phenomenon and an application of our results to
theories of massive gravity are discussed.
