Constituent structure Conclusion
We have introduced two concepts which are crucial for understanding the structure of sentences: constituents and categories.
Syntactic constituents are groups of words that function as a unit in terms of word order: they may replace or be replaced by a single word, occur together in various positions in a variety of sentence types, be the “focus” of a con tent question, or function as the answer to such a question. A constituent is represented in a tree diagram as a string of words that is exhaustively dominated by a single node.
Syntactic categories are classes of words (or phrases) that share certain properties. Category names appear in a tree diagram as the labels for each non-terminal node. We have said that lexical category names (the traditional names for parts of speech) are assigned on the basis of semantic factors, and that phrases share the same category as their head. However, category membership for both words and phrases must be determined on the basis of shared grammatical properties. Two of the crucial types of evidence are (i) sameness of distribution, and (ii) sameness of internal structure. For lexical categories (words), we ask: (i) what kinds of phrases does this word occur in, and what is its function within the phrase (e.g. head, modifier, complement)?; and (ii) what kinds of affixes occur on this type of word? For phrasal categories we ask: (i) where may this type of phrase occur in a sentence, and what is its function in the sentence?; and (ii) what kinds of words occur, and what is their function, within this type of phrase?
Phrase Structure trees are a popular and efficient way to represent these aspects of sentence structure. However, there are other aspects of syntactic structure that are not directly represented in simple tree diagrams of the sort we have seen thus far.
