In a recent study we have shown that the elastic modulus of paper is controlled by two separate factors - the elastic properties of the fibrous material and the structure of the sheet. Structure controls modulus through the orientation distribution of the fibres and the efficiency of stress transfer between them. These ideas are now extended to the plastic regime. It is concluded that the nonlinearity of the stress-strain curve and the viscoelasticity of paper are primarily properties of the fibres. In some cases fibre-fibre bond breakage during straining can reduce the efficiency of stress transfer sufficiently to make a contribution to the nonlinearity of the curve.