Halliday & Matthiessen (1999: 178):
Relators are typically realised by conjunctions, like and, so, if, that, because, however; these can form groups, such as as if, and yet, but conjunctions are more often expanded by adverbs (just because, even if). In addition, there are numerous other types of relator: prepositional phrases (in addition, in the event (that), for fear that), nominal groups remaining from earlier prepositional phrases ([at] the moment (that), [on] the day (that)), and various expressions involving non-finite verbs (supposing (that), provided (that)). The relator construes a logico-semantic relation between the clauses in a clause nexus (realising a sequence), but it is itself an element in the structure of one or other of the two clauses concerned;