Halliday & Matthiessen (1999: 23):
Consider a series of examples such as the following:
What happens here is that a meaning has gradually crystallised, as it were, out of the total meaning potential of the system so that it can be deployed in codified form instead of being constructed afresh each time. In an animal that has four legs, each of the component elements animal four & legs is codified separately, as are the various grammatical relations involved; but the complex is not codified as a whole. When we come to quadruped, it is. Again, this codifying progression takes place in all the three dimensions of history: quadruped evolves later in the system, is learnt later by a child, and typically at least appears later in the text (cf. an animal that has four legs is called a quadruped).