Because the elasticity of a material is described in terms of a stress–strain relation, it is essential that the terms "stress" and "strain" be defined without ambiguity. Typically, two types of relation are considered. The first type deals with materials that are elastic only for small strains. The second deals with materials that are not limited to small strains. Clearly, the second type of relation is more general in the sense that it must include the first type as a special case.