Halliday & Matthiessen (2014: 25):
When we say that language is stratified in this way, we mean that this is how we have to model language if we want to explain it. A language is a series of redundancies by which we link our ecosocial environment to nonrandom disturbances in the air (soundwaves). Each step is, of course, masterminded by the brain. The relationship among the strata — the process of linking one level of organisation with another – is called realisation.