Halliday & Matthiessen (2014: 25):
It might be asked whether an analogous stratification took place within the expression plane; and the answer would appear to be ‘yes, it did’, and for analogous reasons, namely separating the organising function from the function of interfacing with the environment. Here, however, the environment is the human body, the biological resource with which sounding (or signing) is carried out. Taking sound (spoken language) as the base, the stratification is into phonetics, the interfacing with the body’s resources for speech and for hearing, and phonology, the organisation of speech sound into formal structures and systems.