The Guiding Principle Of Compositional Hierarchies: Exhaustiveness

Halliday & Matthiessen (2014: 21-2):

The guiding principle is that of exhaustiveness: thus, in the writing system, a word consists of a whole number of letters, a sub-sentence of a whole number of words, a sentence of a whole number of sub-sentences; the number may be more than one, or just one. At the same time, as always in language, there is much indeterminacy, or room for manoeuvre …