Equal temperament is the standard tuning for instruments in the West and has shaped our current way of thinking about music. The purpose of this research is to explore how the knowledge put to use and the meanings associated with the historical emergence of equal temperament, in instruments that preceded the guitar, can contribute to an interdisciplinary approach to learning mathematics and music. For this, an interpretative analysis of the content of the book Mersenne’s Universal Harmony of 1637 was carried out, and the process of genesis and development in the 16th century of the knowledge present in this book was studied. The results reveal that it is possible to rethink the teaching of some geometric contents at school level, such as Euclid’s theorem and geometric progression, based on the problem of the proportional division of the guitar neck. We also conclude that the notion of self-similarity would allow the generation of interdisciplinary didactic proposals that foster a transversal link between Mathematics and Music.