Halliday (2008: 174):
The notion of grammaticalisation depends on the notion of a closed system. This has the properties:
- a small fixed set of possibilities, x/y/z…, which are
- mutually exclusive, x = “not y or z”, and
- strictly proportional, x : y : z, is constant, with
- a defined condition of entry, eg nominal group, and
- generalised to a large domain of application, eg every major clause