A theory is described which relates the Selwyn granularity coefficient to emulsion structure. The emulsion grain size distribution is taken into account, which leads to the result that in a plot of the type In G = a + b In D where G = Selwyn granularity coefficient, D= average optical density, the value of the constant b must be less than 1/2. It is shown that the granularity of different films cannot be simply correlated with sensitometric properties.