Resumen
In this paper we examine a corpus of sixteenth- to nineteenth-century Basque private letters and administration documents from a historical sociolinguistic point of view. Because of the diglossic situation, such texts are rare but valuable in the historical corpus of Basque. In particular, we analyse relative clauses with the pronoun zein ‘which’. The construction was borrowed from Romance by highly literate bilinguals, but then spread, especially through formulaic language, to less literate writers too. We focus on the development of non-standard variants of this relative clause, which are common in our corpus, but not found in printed literary texts. We argue that the sociolinguistic context was crucial in the emergence, spread and syntactic change of the relative construction.