  A formalism for describing an all-sky map of the polarization of the cosmic
microwave background is presented. The polarization pattern on the sky can be
decomposed into two geometrically distinct components. One of these components
is not coupled to density inhomogeneities. A non-zero amplitude for this
component of polarization can only be caused by tensor or vector metric
perturbations. This allows unambiguous identification of long-wavelength
gravity waves or large-scale vortical flows at the time of last scattering.
