Halliday & Matthiessen (1999: 320):
The context encompasses both the field of activity and the subject matter with which it is concerned (‘what’s going on, and what is it about?’) and the tenor of the relationship between the interactants, between speaker and listener, in terms of social rôles in general and those created through language in particular (‘who are taking part?). The field is thus the culturally recognised repertoires of social practices and concerns, and the tenor the culturally recognised repertoires of rôle relationships and interactive patterns.