<![CDATA[Learning mathematics on the topic of geometric transformations of lines still faces challenging because the concepts are abstract, making it difficult for students to understand changes in the position of object on the Cartesian coordinate plane. This study aimed to develop the Cartesian coordinate monopoly (MONIKA) teaching aid and to determine its quality in terms of validity, practicality, and effectiveness. This study employed a Research and Development (R&D) approach using the Hannafin and Peck model, which consists of three stages: needs assessment, design, and development and implementation. The research subjects were ninth-grade junior high school students. The research instruments included material and media expert validation sheets, and pretest and posttest instruments. Data were analyzed using percentage analysis for validity and practicality, as well as the Normalized Gain (N-Gain) test. The development process involved needs assessment, design, and development and implementation stages. The results showed that MONIKA obtained a validity score of 92.5% from the material expert 93.75% from the media expert, both categorized as “HighlyValid”. In terms of practicality, the product was categorized as “Highly Practical” based on student response questionnaires and learning implementation observations. Meanwhile, the effectiveness test yielded an average N-Gain score of 0,3746, which falls into the moderate category. Therefore, the MONIKA teaching aid is considered valid, practical, and effective, making it suitable for use as a mathematics teaching aid for learning geometric transformations of lines.]]>
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