Halliday & Matthiessen (1999: 527):
Some particular interpersonal colouring may inform the whole of an individual speaker's interaction with another person; and because human societies are inherently hierarchical, the interpersonal component of the grammar in many languages enacts networks of social relationships with varying degrees of inequality and of distance. Thus there may be regular lexicogrammatical variants used to maintain different alignments of speaker and listener, and even of third parties, on vectors of power and familiarity; such forms may be located at one point in each grammatical structure (for example in the endings of the verb) or dispersed prosodically throughout the wording of the clause.But even in a language such as English, where there are no such systematic speech styles institutionalised in the grammar, there is always some functional variation along these lines: we have no difficulty in recognising what are the more formal and what are the more informal variants among different samples of spoken or written discourse.