12 Phrase structure
That sentences and phrases in human language have abstract hierarchical structure, not merely sequences of words and formatives, is one of the fundamental discoveries of modern linguistics. Accordingly, any theory of human language must have a component or device that deals with its “phrase structure”, regardless of the analyses it offers for other properties of language (such as transformations). In this sense, the theory of phrase structure is a kind of backbone for contemporary linguistic theory.
In earlier generative traditions, the properties of phrase structure were coded in terms of the formal mechanism called “phrase structure rules” of the following form, where α is a single symbol and φ, ψ, and ...