The development of the future and conditional tense in Castilian – as in Romance in general – has been universally acknowledged as a typical case of grammaticalisation, whereby the two components of the Latin periphrasis [infinitive + HABERE] fused into a synthetic form (cantaré, cantaría). In Old Castilian variation between synthetic forms and analytic forms (cantar lo é, cantar lo ía) can be witnessed, which indicates that the grammaticalisation process is still ongoing. The synthetic structures eventually triumphed over the analytic ones in the Early Modern period, thereby completing the grammaticalisation process. The literature on this topic shows an important gap as the exact chronology and the geography of this development remain largely unexplored. A comparison between Castilian and its neighbour languages (Galician-Portuguese, Asturian-Leonese, Navarro-Aragonese, Catalan, Occitan) hints at a grammaticalisation continuum that covers the Iberian Peninsula and beyond. A recently formulated hypothesis proposes that the spread of the change occurred through language contact following a North-South-West route. The proposed research will examine and test this hypothesis and aim to establish the geography of the grammaticalisation process while investigating the concrete roles of grammaticalisation and language and dialect contact in this structural convergence phenomenon.