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Description: Permeability is an increasingly prevalent metric included in volcano modelling; however, it is a property that can exhibit anisotropy in volcanic environments. Permeability of a layered medium can be described by the arithmetic or harmonic means of the permeabilities of the constituent units, depending on the orientation of flow with respect to layering (i.e. flow parallel or perpendicular to layering, respectively). We outline the theory underlying these formulations, and provide experimental permeability data measured on anisotropic volcanic materials in order to demonstrate this point. We highlight that permeability measured parallel to layering or bedding must be higher than that measured perpendicular to layering. Moreover, we emphasise that the choice of averaging method used to upscale permeability data (i.e.to calculate the equivalent permeability of a system) has important consequences on the validity of the derived values. We anticipate that these points will help move towards more realistic models of pressure evolution behaviour in volcanoes, and increase the utility of laboratory-derived data for volcano-scale modelling.