Andean linguistics has not determined the geographic extent in which Quechua was spoken in Peru's Cajamarca region during the colonial period. The debate centers on whether it ranged beyond where it is currently found in the ‘enclaves’ of Porcón and Chetilla. No previous systematic efforts have attempted to clarify this problem. Here, we seek to reconstruct 17th-century Quechua distribution using data from trial interpretations (oral language translations carried out during the testimonies) in the document series ‘Protector de Naturales' (‘Advocate of the Indians’), held in the Regional Archive of Cajamarca. We represent this data cartographically using the dot density map technique, a visualization method that allows us to conclude that in the 17th century Quechua covered a wider territory than that which is currently observed. We suggest that this method could be applied in other contexts, to increase knowledge of the historical distribution of indigenous languages in South America.