the referring phrase that denotes the necessarily shared participant in a relative clause construction – that is, the participant that plays a semantic role in both the event denoted by the relative clause and the event denoted by the matrix clause. Example: in I ate the cheesecake [that Carol baked], the cheesecake is the relative clause head. The relative clause head is an argument of the matrix clause predicate, and is modified by the relative clause. (Section 19.1)