Halliday & Matthiessen (1999: 520):
The changes that constitute our experience are not all independent of one another. On the contrary; our experience is that one thing leads to another, and there is in principle no limit to an experiential chain. But the exact nature of the relationship may vary from one transition to another; so the grammar construes the relationship between processes dyadically, in the form of a nexus between a pair of clauses. The first process may have a second process related to it, by a relationship such as sequence in time, or cause and effect; this in turn may have another one related to it, either by the same relation or a different one — in either case, the relationship is construed as holding between members of a pair.