Abstract This paper elucidates the role of internal and external factors in the semantic evolution of Basque zu from ‘you.pl’ to exclusively ‘you.sg’ and in the development of Basque zuek ‘you.pl’. Transparently, zuek involves suffixation of the demonstrative hek ‘those’ to zu. The neighboring Romance languages exhibit a parallel evolution that involves the adjectival collocation vōs alteros ‘you others’, but other similar constructions are recorded. This paper defends the premise that Basque zuek emerged through a pattern replication process in which the pivotal feature was the morphological expression of number. We argue that the grammaticalization of zuek as ‘you.pl’ started in eastern areas due to contact with Occitan, a language in which grammaticalized ‘you.pl’ forms appear earlier than in Castilian Spanish. We also address an innovation exclusive to Basque: a series of second plural verbal forms which exhibit a geographically spread pattern within the Basque Country similar to that of the Romance innovative pronouns in the Iberian Peninsula.