The paper demonstrates not only the usefulness but also the limitations of relative density by three examples from research and practice. The first example deals with the problem of relating the angle of internal friction to relative density. When sands do not differ significantly in their grain-size distributions, the parameters can be related. However, for a large variety of sands, compactibility has to be taken into account as a third parameter. The second example describes the use of relative density in subsoil exploration. For a specific case, the number of blows of the standard penetration test, relative density, friction angle, and density are empirically related to each other, enabling the designer of a harbor project to reduce soil investigation costs. In the third example, performance of pneumatic tires tested in two different sands is different when related to cone penetration resistance. However, when cone penetration resistance is replaced by relative density, performance is comparable for tests conducted in the two sands at the same relative density.