Halliday & Matthiessen (1999: 29-30):
In the ideational domains, the representation is usually conventional; but even here there is a relationship of analogy, where we find in the sounding, modes of organisation similar to those of wording (and therefore of meaning). Systemically, we find that the system construes a phonetic space — notably the vowel space; and that this provides a model for semantic space. Structurally, we find that sound is structured both as [logical] chains of segments (e.g. rhythmic units interpreted as syllable complexes) and as [experiential] configurations of segmental constituents (e.g. syllables interpreted as configurations of phonemes).