<![CDATA[In Euclidean geometry, parallelism is a fundamental concept from which many other geometric ideas are derived. Although it is central to geometry and essential for fostering deductive and logical reasoning, research on students’ understanding of parallelism—particularly studies that examine learning obstacles and didactical approaches to address them—remains limited, especially at the tertiary level. This study investigates how students construct theorems of parallelism conditions by identifying learning obstacles and implementing a GeoGebra‑based didactical design within the framework of the theory of didactical situations. Adopting a qualitative approach with a phenomenological–hermeneutic design, the study first examined 64 mathematics education students at a state university in Aceh, Indonesia, to identify learning obstacles related to theorems of parallelism conditions, and subsequently involved 35 of these students in the implementation of the didactical design. Data sources included written documentations, interviews, classroom observations, and audio‑video recordings. Data analysis followed Miles and Huberman’s procedures of data reduction, data display, and conclusion drawing. The analysis showed that students’ understanding of theorems of parallelism conditions was constrained by three main categories of learning obstacles: epistemological, didactical, and ontogenic. In response, a GeoGebra‑based didactical design was developed and implemented. The findings indicate that this design effectively mitigated the identified obstacles and supported students’ construction of theorems concerning conditions for parallelism. Moreover, the design contributed to a shift in students’ learning from predominantly procedural engagement toward more conceptual understanding, although this shift was not yet fully uniform across all students. These results underscore the importance of integrating carefully planned didactical situations into the teaching of geometry and of employing dynamic visualization tools such as GeoGebra to support students’ transition from empirical reasoning toward more deductive mathematical understanding.]]>
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