Short-term and sudden changes in mathematics program, removing some topics without paying attention to key concepts, may break ties between subjects and concepts. For this reason in research, it has been aimed to investigate whether the set is a key concept, from the concept of set extracted from the primary and secondary school mathematics curriculum, whether the issues affected by the notion of the set are not in the curriculum and whether or not the set takes place in the curriculum. This study is a case study in the qualitative research method. The study group is. Five classroom teachers, 12 elementary mathematics teachers and five education of mathematics academicians from universities Three of the academician are professors and one of them is associate professor, and the other one is doctor. These participants were selected from teachers and faculty members working in northeastern Turkey. Subject-focused interviews were conducted with questions prepared for standardized open-ended interviews during the collection of research data. Negotiations were recorded with voice recorder. These audio recordings were transcribed through the Microsoft Word writing program. Descriptive analysis on qualitative research methods was conducted on interviews. According to twenty- two participants, set is a key concept for mathematical topics. Two participants express that the set is not a key concept. As a result of interviews with the participants, issues have been influenced by the absence of the set concept. In addition, 20 participants sustain that the set should be in the curriculum, but two participants sustain that set should be in the curriculum. Accordıng to view of participants, set is a key concept and the absence of the set mathematical subjects will be ınfluenced. For this reason, set should be in currıculum.