The relationship between the conjunctive coordinator ‘and’ and the disjunctive coordinator ‘or’ in natural languages constitutes the theoretical framework for this research, which examines its expression in Latin, medieval Castilian and Basque. The study reveals two disjunctive coordinators which have so far not been registered in literature on the subject, the Latin pleonasm et aut and the medieval Castilian do, and we analyse their relationship with the Basque edo, still very much alive today, but whose origin has not so far found a satisfactory explanation. The theoretical arguments and empirical data allow us to defend the historical sequence – Latin et aut > medieval Castilian (e)do > Basque edo –, and to focus attention on the problems posed by the conceptual dichotomies conjunction/disjunction and inclusive/exclusive disjunction for the description of semantic relationship contained under the term «coordination»

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