The notion of comparison class has figured prominently in recent analyses of the gradability properties of adjectives. We assume that the comparison class is introduced by the degree morphology of the adjective and present a new proposal where comparison classes are crucial to explain the distribution of adjectives in Spanish copular sentences headed by the verbs "ser" (be/SER) and "estar" (be/ESTAR). The copula "estar" (be/ESTAR) appears whenever a gradable adjective merges with a within-individual comparison class, a modifier expressing a property of stages. The copular verb "ser" (be/SER) appears when a gradable adjective merges with a between-individuals comparison class, a modifier expressing a property of individuals. The distinction between relative and absolute adjectives can be reduced to the semantic properties of the modifier expressing the comparison class that is merged in the functional structure of the adjective.