The paper from A. Hansen, S. Sinha, D. Bedeaux, S. Kjelstrup. M. Aa. Gjennestad and M. Vassvik: “Relations between seepage velocities in immiscible, incompressible two-phase flow in porous media” (2018) has become the most downloaded paper ever in Transport in Porous Media, the main journal in porous media research, starting in 1986. We demonstrate in the paper how certain easily overlooked symmetries in immiscible two-phase flow in porous media lead to new equations constraining how the two fluids move. (A symmetry is a property that is unchanged after transforming the system.) The ultimate goal of this work is to be able to present a complete theoretical description of immiscible two-phase flow in the limit where the porous medium may be seen as a continuum, i.e. the scales where relative permeability theory today reigns.