This paper describes a study about inter-understanding between Portuguese and Spanish. Based on Peano’s work, the study resorts to the logical relation of intersection between sets and applies it to words in Portuguese and Spanish meaning the same. The material is a text that is the abstract of a psychology paper. Each of the words in that text, which is written in Portuguese, is compared to its corresponding Spanish word by means of the relation of intersection. This is made by (...) considering the letters in the words to be elements in sets. The results show the potential that this methodology can have to provide percentages of inter-understanding between the two languages (and between any pair of languages). (shrink)