This study tries to identify high school students' knowledge about the concept of proof, based on classroom discussion. The processes of discourses, both natural and prompted, are studied as they occur between students and teachers. The study employs discourse analysis as the qualitative research framework. Participants are 13 Science High School students from Izmir (Turkey) in 11th grade and two mathematics and geometry teachers. Data gathered consist of 53 filmed mathematics and geometry classes, recorded over three months, plus researchers' field notes. The focus of the study is on verbal discourse in the classroom between teachers and students. 18 recorded discourses were analyzed after transcription. The theoretical framework of the study is dependent on the social semiotics, with Halliday's Systemic Functional Linguistics model (SFL) used in the analysis. In the SFL model there are three main components; field of discourse, tenor of discourse, and mode of discourse. This study presents findings and analysis results based on the field of discourse. Emergent findings showed important effects of teacher-student in-class discourses (in terms of the structure, diversity, and pattern characteristics) on the students? learning about proof and knowledge constructions.