Suppose that we have a sequence of rewriting rules [...] including in particular the rule (8) 'A' →'X'. Associated with this rule is each grammatical function (9) ['B', 'A'] where 'B' is a category and 'X' = 'YBZ', for some 'Y', 'Z' (possibly null). Given a Phrase-marker of the terminal string 'W', we say that the substring 'U' of 'W' bears the grammatical relation ['B', 'A'] to the substring 'V' of 'W' if 'V' is dominated by a node labeled 'A' which directly dominates 'YBZ', and 'U' is dominated by this occurrence of 'B'.
- Chomsky (1969), a pag.70-71