  We derive a simple consistency relation from the running of the
tensor-to-scalar ratio. This new relation is first order in the slow-roll
approximation. While for single field models we can obtain what can be found by
using other observables, multi-field cases in general give non-trivial
contributions dependent on the geometry of the field space and the inflationary
dynamics, which can be probed observationally from this relation. The running
of the tensor-to-scalar ratio may be detected by direct laser interferometer
experiments.
