It is shown that, in four dimensions, it is possible to introduce coordinates so that an analytic metric locally takes block diagonal form. i.e. one can find coordinates such that $g_{ \alpha\beta} = 0$ for $(\alpha, \beta) \in S$ where $S = { (1, 3), (1, 4), (2, 3), (2, 4)}$. We call a coordinate system in which the metric takes this form a 'doubly biorthogonal coordinate system'. We show that all such coordinate systems are determined by a pair of coupled second-order partial differential equations.
Categories: math.DG gr-qc