Antonio de Nebrija (1444?–1522) published his Gramática Castellana in 1492, at a time when humanist appreciation of Castilian as a cultural language had not yet advanced to a discussion of its possibilities to become an established norm. However, an analysis of Nebrija’s linguistic and grammatical theories does shed some light on this question. For instance, it becomes clear that the new method which he proposes for the teaching of Latin (nova ratio Nebrissensis) presupposed a recognition of the presence of universal grammatical concepts in the pupil’s mother tongue. Such a conception is possible because Nebrija accepts an essential starting point of the medieval speculative tradition: language composition may be reduced to two basic concepts: materia (lexical element submitted to ‘corruption’) and forma (other elements – ‘accidents’ – which are stable). This composition is common to all languages. Therefore, Nebrija holds that by making use of the constrastive method it is possible to study two languages such as Latin and Castilian (which also happen to be closely related). Therefore, we must not consider the Gramática Castellana as separate from the rest of Nebrija’s scholarly production. He himself had coined the notion of ‘unity in diversity’ concerning his grammatical work. In order to teach the Castilian language and, starting from Castilian, Latin, Nebrija writes grammatical and lexicographical works which have an underlying unity. His general approach was exclusive to Nebrija; however, although nobody before him had worked out such an ambitious project, there is no doubt that he was continuing on the way in which grammatical tradition had been heading for some time. An example of this tradition is the so-called Grarnmatica proverbiandi. In this paper, the main features of this kind of medieyal grammar are analyzed. It is argued that they constitute the immediate precursor of the Nebrija’s undertaking, since we find in them didactic postulates which he developed further. These postulates led Nebrija to a contrastive grammar of Latin and Castilian and the creation of a grammatical terminology for the vernacular.