This study reports the classroom mathematical practices proposed in an instructional sequence grounding on the principles of inquiry-based learning (IBL) through the use of the dynamic geometry software (DGS) in learning to teach geometric transformations. A 5-week instructional sequence consisting of IBL activities that were guided by the hypothetical learning trajectory (HLT) was designed. The participant 23 preservice middle-school mathematics teachers (PMSMTs) used the geometer's sketchpad in the sequence. The qualitative data obtained from whole-class discussions, classroom artifacts, and teacher candidates' products were analyzed by the three-phase methodology of Rasmussen and Stephan based on the Toulmin argumentation model. The findings revealed that three mathematical practices emerged at the end of the 5-week instructional sequence: (1) an in-depth understanding of geometric transformations, (2) operational definitions of geometric transformations and the identification of the relationship among them, and (3) the properties and attributes of geometric transformations. The development of the PMSMTs' understanding of geometric transformations in the instructional sequence was explained by the systematic nature of classroom mathematical practices. Hence, it was concluded that the PMSMTs' knowledge and understanding of transformation geometry were enhanced through the instructional sequence based on the specific HLT.