  We explore the consequences of a detection of primordial tensor fluctuations
for general single-field models of inflation. Using the effective theory of
inflation, we propose a generalization of the Lyth bound. Our bound applies to
all single-field models with two-derivative kinetic terms for the scalar
fluctuations and is always stronger than the corresponding bound for slow-roll
models. This shows that non-trivial dynamics can't evade the Lyth bound. We
also present a weaker, but completely universal bound that holds whenever the
Null Energy Condition (NEC) is satisfied at horizon crossing.
