This study aimed to analyse the phenomena from didactic transposition process and Hypothetical Learning Trajectory (HLT) in material of method to describe a set. A qualitative method with an interpretive phenomenology analysis approach was used to reveal this didactic phenomenon. A mathematics teacher and 28 students of 7th grade had been participants in the study. Furthermore, the data analysed included the results from review of scholarly mathematics textbook and school mathematics textbooks,

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... observation of the teaching and learning process, diagnostic test, and student interview, and these were analysed using qualitative analysis. The result of this study shows that the transposition of knowledge from mathematical scholarly knowledge to taught knowledge indicate a shift in concept. The flow of material presentation and student learning trajectory in both school mathematics textbooks and the teaching and learning process had weaknesses, which were causing learning obstacles to students. This learning obstacles had an impact on students' difficulties in building conceptual understanding of the method to describe a set material. And then, the findings of didactic transposition analysis become the basis for compiling a new HLT which will serve as a framework in designing didactic situations in the form of didactic designs. So, the didactic transposition analysis and HLT are two things that complement each other in the preparation of didactic situation.