This study explores how hard of hearing students decided whether the shape was a polygon and which semiotic sources were used when the students engaged in explaining geometrical concepts. It was defined how the students interacted with geometric shapes using semiotic sources and examined how such multimodal interactions with geometric figures displayed their reasoning. The study was a case study and carried out three hard of hearing students. The data was collected through interviews and analyzed with content analysis. It was detected that the students paid attention to edge, angle, and vertex of the shapes in the process of identifying polygon. It was seen that the students used gesture, speech, sign language, inscriptions which are semiotic sources and personal or mathematical definitions to express polygon concept. However, it has been determined that students have some misconceptions in the process of explaining concepts. It is suggested that the words used in the concept definition should be selected carefully by the teachers to teach the concepts correctly and the teachers use hand signs for concepts in their lesson.