This article draws from data produced during subject theory lectures and in conversional interviews with students from an ongoing ethnographic study of mathematics teacher education at a Swedish University. Using Bernsteins’ language of description of the pedagogic device the article describes how the aims of teacher education to re-contextualise mathematical education towards greater student subject knowledge is thwarted by a strongly classified and framed practice that obstructs student teachers from developing a vertical knowledge structure in mathematics due to performative priorities. The mathematical knowledge to which students are subjected takes more the form of a horizontal discourse and this is problematic for their knowledge development. A horizontal discourse reduces student access to important forms of knowledge by which they can challenge tradition and consciously change their practice.