Mobile armor layers which form during bed load transport of non-uniform sediments are shown to be closely related to the static armor layers that form by selective erosion as a result of the action of clear water flows. Two previously published numerical models of the transport of non-uniform sediments are used as a basis for the discussion. Each model is inverted, so that it predicts surface layer compositions given the imposed flow conditions and bed load transport rate and composition. Static armor is then obtained in the limit of vanishing sediment transport, under the constraint that the bedload and substrate size distributions are identical. Good agreement is obtained between measured and calculated static armor compositions over the entire grain size distribution.