Halliday & Matthiessen (2014: 603):
… there is a set of lexicogrammatical systems that have evolved specifically as a resource for making it possible to transcend the boundaries of the clause – i.e. the domain of the highest-ranking grammatical unit. These lexicogrammatical systems originate in the textual metafunction and are collectively known as the system of COHESION. … There are four ways by which cohesion is created in English: by (i) conjunction, (ii) reference, (iii) ellipsis, and (iv) lexical organisation.