The relative size and roughness of a stable armor varies with particle shape. For a given imposed shear stress the order of increasing nominal particle diameter is flat, angular, and rounded gravel. The order of increasing surface roughness is flat, rounded, and angular gravel. The mobility of fine flat gravel particles is enhanced by the lower surface roughness of armors which exhibit an imbricated structure. However, the generally lower mobility of ellipsoids means that, for a given stream power, bed load discharge associated with armor development on flat gravel is less than that for rounded gravel (at greater depth to grain size ratios). The larger projected area of angular gravel enhances the mobility of coarse grains at high flows, offsetting interlocking effects which restrict the mobility of angular gravel at low flows. Relative and absolute size effects contribute to the departure from equal mobility of coarse and fine particles that is a ubiquitous feature of stable armor development below the limit for the formation of a threshold armor when all particles are equally mobile. The transport of fine sediment as throughput load may mask the effect due to hiding in the field and give the erroneous appearance that stable armors develop solely through winnowing. All particle sizes in the underlying bed material are present in the armor and in the bed load, and coarsening is but one of several adjustments to the surficial bed material that occur as a stable armor evolves. It likely involves the wholesale rearrangement of surficial particles and may entail minimization of shear due to drag on background roughness. ¿ American Geophysical Union 1994