Structural Hierarchy: the Grammar Ranks

  Texts are written or spoken following a certain structure so that readers or interlocutors can understand them. If we focus on written texts, they are divided into paragraphs. Every paragraph is made up of a number of sentences. Each sentence contains one or more clauses. Each clause is made up of different phrases. Each phrase consists of one or more words. Each word is made up of one or more morphemes. 

This structural hierarchy constitutes what Halliday (1961) calls the rank scale of Grammar. Within this rank scale, sentences and clauses comprise the highest structural units of syntax. Besides, each clause can be divided into subject and predicate.