This article presents a Vygotsky-inspired analysis of how a teacher mediated a "thinking aloud" whole-group discussion in a 6th grade mathematics classroom. This discussion centered on finding patterns in a triangular array of consecutive numbers as a phase towards building recursive and direct algebraic formulas. By a "thinking aloud" discussion we mean a conversation wherein students exchange and further develop ideas-in-the-making with their peers under the teacher's guidance. Drawing upon Halliday's systemic functional linguistics (SFL), we treated the selected discussion as a text. We then analyzed how the teacher mediated the conjoined making of this text so that it served as an interpersonal gateway for students to practice searching for patterns and signifying these patterns in propositional form. This analysis was guided by the following questions: How did the discussion as a text-in-the-making mean what it did? What was the role of the teacher in the conjoined making of this text? Our study illustrates the power of SFL for capturing the inner grammar of instructional conversations thus illuminating the complexities and subtleties of the teacher's role in mediating semiotic mediation in mathematics classrooms.