This dissertation provides a typological account of the cross-linguistic functions that can be associated with periphrastic 'do'-constructions minimally consisting of a lexical verb and a ·schematic action- auxiliary element equivalent to English do. The investigation is based on a sample of 200 genetically and typologically diverse languages.
While in some languages the occurrence of a periphrastic ‘do' -auxiliary in the clause is obligatory under certain morphosyntactic conditions, it is optional in others. The analysis of periphrastic 'do' -constructions in the sample shows that the variation of functions associated with obligatory 'do'-periphrasis as a consequence of grammatical properties is limited and can be described in terms of four types, A considerable number of languages that employ 'do' -periphrasis optionally allows similar form-function relations, which suggests that these four types represent likely domains for grammaticalisation. Functions of optional 'do’-periphrasis nevertheless also include the establishment of discourse coherence and morphosyntactic economy, more specifically avoidance of morphological complexity, for instance during language acquisition.
The sample also contains languages in which a 'do’--auxiliary directly encodes grammatical categories typically associated with verb morphology such as tense, mood and aspect. In these languages the ·do' -auxiliary is not triggered by morphosyntactic features of the clause it occurs in. Furthermore 'do' -periphrasis can be attested as a cross-linguistically common strategy for the purpose of symmetric verb coordination.