Context-Free Grammars and Constituency Parsing

19.9 Heads and head-finding

1. What is the lexical head of a constituent?
   The grammatically most important word of the phrase (N for an NP, V for a VP). Heads are passed up the tree, so each non-terminal is annotated with its head word; this idea dates to Bloomfield (1914).
   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 19, §19.9, p. 444
2. Why are heads important?
   They are central to dependency grammar and can map between constituency and dependency parses; they matter in probabilistic parsing and formalisms like HPSG.
   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 19, §19.9, p. 444
3. How are heads found in practical systems?
   Dynamically after parsing, by walking the tree with hand-written head rules (a head percolation table, Magerman/Collins), e.g. for NP: return a final POS; else search right-to-left for an NN, NNP, NNPS…; else left-to-right for an NP; and so on.
   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 19, §19.9, p. 445