Unbounded dependency constructions (UDCs)
UDCs is a larger class of constructions, to which relative clauses also belong.
Unbounded dependency constructions are those which contain an anaphoric gap, with no upper bound,and no matter how deeply embedded this gap may be.
There is a set of examples to show what is meant by an UDC:
1. It was the day, [she would always remember__]. 2. It was the day, [I think [she would always remember__]]. 3. It was the day, [I think [you said [she would always remember__]]].
The outer brackets enclose the relative clause, while the inner ones enclose the nucleus. There is a gap in the nucleus in the position of the object of the verb (remember). The gap derives its interpretation from 'the day'. So, we can approve, that the gap is anaphorically linked directly to the antecedent.
If there were the relative phrase in prenuclear position, then the gap would be linked first to this relative phrase. In this case relative pronoun derives its interpretation from the antecedent and then the gap takes it from the phrase, so that the relative pronoun play the role of mediator, like in the following:
It was the key, [that [I knew [he looked for_]]].
The gap and the relative phrase 'that' is a dependency relation. The dependency between the gap and antecedent is unbounded in the sense that there is no upper bound, or limit, on how deeply embedded within the relative clause the gap may be.