  We study the evolution of the metric perturbations in a Bianchi background in
the long-wavelength limit. By applying the gradient expansion to the equations
of motion we exhibit a generalized "Separate Universe" approach to the
cosmological perturbation theory. Having found this consistent separate
universe picture, we introduce the $\delta M $ formalism for calculating the
evolution of the linear tensor perturbations in anisotropic inflation models in
{\it almost} the same way that the so-called $\delta N$ formula is applied to
the super-horizon dynamics of the curvature perturbations. Similar to her twin
formula, $\delta N$, this new method can substantially reduce the amount of
calculations related to the evolution of tensor modes. However, it is not as
general as $\delta N$, it is a "perturbative" formula and solves the shear only
to linear order. In other words, it is restricted to weak shear limit.
