The upper bound of the semantic stratum is, as we have said, the text: this is the most

extensive unit of meaning. The upper bound of the lexicogrammatical system is the clause: this is the most extensive unit of wording. * In the grammar, there is a single, generalized compositional scale - the grammatical rank scale introduced in Chapter 1, Section 1.1.3, p.7 (clause - group/phrase - word - morpheme); and we can specify not only the upper and lower bounds of this scale - the clause and the morpheme, respectively - but also the intermediate units of patterning: group/phrase and word. But in the semantics, it is far from clear whether there is a single compositional scale that is generalized across all registerial varieties of a language. It is quite possible that different registers operate with different compositional scales; for example, one such scale was identified in the organization of classroom discourse by Sinclair and Coulthard (1975) and another in the organization of certain types of conversation by Cloran (1994). This issue can only be settled after a great deal more research into the semantics of text has been carried out (for discussion, see Matthiessen and Halliday, in prep).