Using a constructional approach to morphosyntax, this study describes a triclausal construction (a type of anankastic conditional construction) and related constructions in the history of Chinese. It demonstrates that the triclausal construction constitutes a context of morphosyntactic vagueness where category boundaries between modals and conditional protasis connectives are underdetermined; consequently, bidirectional rather than unidirectional developments occur. Morphosyntactic vagueness is defined by properties shared between two morphosyntactic categories: distributional and functional similarities. Therefore, changes enabled by morphosyntactic vagueness are argued to be regular processes of change mediated by grammatical equivalence. If grammaticalization is defined as the development of morphosyntactic categories, but not in terms of non-equivalence such as unidirectionality or increased grammaticality, grammaticalization may be systematically bidirectional when enabled by morphosyntactic vagueness.
