Halliday & Matthiessen (1999: 548-9):
But even non-metaphorical forms of writing construe with greater determinacy. We may cite two very pervasive distinctions between spoken and written discourse. On the one hand, writing construes the text into clear-cut constituents, marked off by spacing and other forms of punctuation; in spoken language there are no clear beginnings and endings in the expression (we cannot refer to pauses, since they tend to occur at transition points before something that is less predictable; pauses seldom mark the text's grammatical boundaries). On the other hand, many interpersonal and textual systems are realised in speech by intonation, and most intonation contrasts are gradual rather than categorical. Thus both syntagmatically and paradigmatically written language tends towards greater determinacy; hence our received model of language, in the mainstream grammatical tradition, emphasises clear-cut constituents and classes. Not that it has no tolerance at all for mixed and intermediate categories; but it treats them as the exception, not the norm.