Many factors affect the strength of what engineers call soil. This report deals primarily with the effect of the grain size of sand or silt particles mixed with the clay in the soil. In order to understand the effect of grain size, the influence of (1) water content, (2) type of clay mineral, and (3) ratio of clay to sand in the soil must be known. The effect of these four variables was investigated by measuring the strength of a series of synthetic soils in which the water content, clay type, clay-sand ratio, and grain size of admixed sand were changed from one experiment to another. For given water content, kaolin and illite are essentially equal in strength, and both are much weaker than mont-morillonite. Ball clay—a kaolin containing organic matter—is intermediate in strength. For all clays the strength increases, for given water content and given grain size, as the ratio of clay to sand increases. For given water content and given clay-sand ratio, the strength increases as the grain size of the sand decreases below 135 microns. For coarser sand, grain size has little effect. The cause of the greater strength for increasing fineness of sand is ascribed to the well-known principle of greater surface area upon which forces can act.