  The 1-D evolution equations for warped discs come in two flavors: For very
viscous discs the internal torque vector G is uniquely determined by the local
conditions in the disc, and warps tend to damp out rapidly if they are not
continuously driven. For very inviscid discs, on the other hand, G becomes a
dynamic quantity, and a warp will propagate through the disc as a wave. The
equations governing both regimes are usually treated separately. A unified set
of equations was postulated recently by Martin et al. (2019), but not yet
derived from the underlying physics. The standard method for deriving these
equations is based on a perturbation series expansion, which is a powerful, but
somewhat abstract technique. A more straightforward method is to employ the
warped shearing box framework of Ogilvie and Latter (2013), which so far has
not yet been used to derive the equations for the wavelike regime. The goal of
this paper is to analyze the warped disc equations in both regimes using the
warped shearing box framework, to derive a unified set of equations, valid for
small warps, and to discuss how our results can be interpreted in terms of the
affine tilted-slab approach of Ogilvie (2018).
