In addition to time and place, which are inseparable from sociolinguistic variation, language may vary according to age, social class, sex or (social) gender, ethnicity, medium, style, and register. Contact between speakers often leads to change, and different patterns result according to whether this contact involves first-language (L1) or second language (L2) acquisition. Thus, ‘family tree’ aspects of language change are largely accounted for by transmission (involving L1 acquisition), whilst ‘wave model’ changes can be explained in terms of diffusion (involving L2 acquisition). Languages with a high degree of L2 contact will tend to simplify, whilst stable bilingualism or isolation will often lead to complexification. Contact may be interlinguistic or intralinguistic, sometimes resulting in complex linguistic repertoires, with up to four different levels existing simultaneously (national standard, regional standard, interdialectal koiné, local dialect). Contact may also result in code-switching, the emergence of contact vernaculars, and ‘language death’. The receptiveness of a variety to contact influence depends on the extent to which its social networks are open or closed and on the social attitudes of its speakers. Standard languages emerge through a variety of conscious and unconscious processes, and attempts may be made to give non-standard speech varieties a distinct linguistic identity through codification and the creation of literature.