Halliday & Matthiessen (1999: 196):
Types of enhancement: reason, time, place, wayGeneral category: between 'semiotic object' and 'abstraction'Particular features: countable, though typically occurring in singular; are names for major circumstantial classes (reason : why; time : when; place : where; way : how); can be qualified by relative clause (without the need for a circumstantial marker did you see the way [that] they glared at us?), varying with clause having corresponding relative adverb (did you see how they glared at us?), and sometimes both together (do you know the reason why they glared at us?).