If the sand contains pebbles, the proportions should be considered in a little different way. Suppose we make a one-to-three mortar with sand that contains ten per cent, of pebbles. We have in reality, then, 3 X .90 = 2.7 parts of sand to one of cement, and .3 part pebbles embedded in this richer mortar. This point is of special significance in making concrete from gravel containing some sand, or from broken stone from which the fine particles or screenings have not been removed. Such fine particles serve to weaken the mortar by increasing the dose of sand, while the proportion of aggregate is diminished. In using aggregates containing some fine material, then, or in using sand containing pebbles or fine gravel, one should not permit himself to be deceived as to the actual richness of the resulting mortar or concrete.