Halliday & Matthiessen (1999: 84):
… from the perspective of the system, we can observe that the basic degree of delicacy is most highly elaborated; from the perspective of the instance, we can observe that it is the most frequent. However, we have to allow for considerable variation within a language — in particular, functional or registerial variation within the system according to the context of use, as in the variation between the spoken system of everyday life and the written system of science …