Halliday & Matthiessen (2014: 7):
… writing is not the representation of speech sound. While every writing system is related to the sound system of its language in systematic and nonrandom ways (exactly how the two are related varies from one language to another), the relationship is not a direct one. There is another level of organisation in language to which both the sound system and the writing system are related, namely the level of wording, or ‘lexicogrammar’. … The sound system and the writing system are the two modes of expression by which the lexicogrammar of a language is represented, or realised (to use the technical term).