The label γ must be constructed from the two constituents α and β. Suppose these are lexical items, each a set of features. Then the simplest assumption would be that γ is either (6) a. the intersection of α and β; b. the union of α and β; c. one or the other of α, β. The option (6a) and (6b) are immediately excluded: the intersection of α, β will generally be irrelevant to output conditions, often null; and the union will be not only irrelevant but "contradictory" if α, β differ in value for some feature, the normal case. We are left with (6c): the label γ is either α or β; one or the other "projects" and is the "head" of K.
- Chomsky (1995), Pag. 244