The linear masking equations, inspired by continuous tone photographic color, were applied by Murray, et al., to halftone printing. This model is considerably simpler than models customarily used for halftone color hardcopy, such as those based on Neugebauer. Despite unrealistic assumptions regarding the image microstructure, the masking equations often perform reasonably well.In this paper, the differences between the linear masking model and the model of Yule and Colt (Neugebauer with wideband Yule-Nielsen correction applied) are parsed. It is shown that, under conditions for which the Yule-Nielsen parameter n grows without bound (or its reciprocal, u, approaches zero), the differences can be attributed to sub-additivity of densities. An approximation is offered to model the sub-additivities.The largest difference between the two models can be expected to occur under normal circumstances when all colorants are printed solid.