Stratal Realisation: Conventional vs Natural

Halliday & Matthiessen (2014: 27):

The realisational relationship between content and expression, more specifically between lexicogrammar and phonology is largely conventional, or ‘arbitrary’ (with certain interesting exceptions relating to prosody and to two areas of articulation, phonæsthesia and onomatopœia). However, the realisational relationship between the two sets of content strata (semantics and lexicogrammar) and the two sets of expression strata (phonology and phonetics) is natural rather than conventional. Patterns of wording reflect patterns of meaning. Part of the task of a functional theory of grammar is to bring out this natural relationship between wording and meaning. The natural relationship between semantics and lexicogrammar becomes more complex and less transparent with the development of lexicogrammatical metaphor … but the relationship is still fundamentally natural rather than arbitrary.