Didactic transposition occurs when scholarly knowledge is transformed into knowledge to be taught in order to reach didactical suitability. In this study, we explore didactic transposition implemented in music lessons from the perspective of didactical suitability. Utilising Design-Based Research, the current study collected qualitative and quantitative data from three urban schools in the Chartered Community of Navarre, Spain. Findings reveal that didactic transposition depends on the teacher’s beliefs because, while one-to-one teaching and academicism are preferred by educators who prioritise music theory, collective teaching and creativity are promoted by teachers who favour holistic development. In conclusion, didactic transposition enables the aspiration to didactical suitability, explaining the characteristics of didactic contracts. Lastly, some pedagogical implications are provided.