The grammatical category ‘declension’ cannot be applied to Welsh substantive nouns since they have one form only for the singular and the plural respectively. But some Welsh grammarians of the 16th and 17th centuries tried to use this category to classify substantive nouns by proposing new definitions, based on the system of plural formation (Robert 1567) or on the system of initial mutations (Rhys 1592; Salesbury 1593). The latter approach formed a short-lived ‘paradigm’ in Welsh grammaticography with a dynamism of its own. It became divorced from the classification of nouns only and was applied to all words which undergo initial mutations (Davies 1621). The history of the definitions of declension in Welsh grammaticography is thus an instructive example of the changes grammatical categories can undergo when applied to a specific vernacular and of the creativity of the vernacular grammarians.