This  paper  considers  why  mathematics  is  used  in  physics.  It  traces  the  use  of  mathe-matics in physics through primary school, junior high school and senior high school in NSW, Australia, considering its role from the point of view of Systemic Functional Linguistics and Legitimation Code Theory. To understand the development of mathematics, two genres that play differing roles in the discipline of physics are introduced: ‘derivation’ and ‘quantifica-tion’. Through an analysis using the concepts of semantic density and semantic gravity from Legitimation Code Theory, these genres are shown to aid physics in developing new knowl-edge and linking its theory to the empirical world. This paper contributes to the growing body of research considering forms of knowledge in academic disciplines and the role of non-lin-guistic semiotic resources in organizing this knowledge.This  paper  considers  why  mathematics  is  used  in  physics.  It  traces  the  use  of  mathe-matics in physics through primary school, junior high school and senior high school in NSW, Australia, considering its role from the point of view of Systemic Functional Linguistics and Legitimation Code Theory. To understand the development of mathematics, two genres that play differing roles in the discipline of physics are introduced: ‘derivation’ and ‘quantifica-tion’. Through an analysis using the concepts of semantic density and semantic gravity from Legitimation Code Theory, these genres are shown to aid physics in developing new knowl-edge and linking its theory to the empirical world. This paper contributes to the growing body of research considering forms of knowledge in academic disciplines and the role of non-lin-guistic semiotic resources in organizing this knowledge.