This paper takes as point of departure the register variable field, and explores its application to the discourse of History and Biology in secondary school classrooms from the perspective of systemic functional linguistics. In particular it considers the functions of technicality and abstraction in these subject specific discourses, and their relation to the high stakes reading and writing expected from students. The paper shows how the practical concepts of power words, power grammar and power composition can be developed from this work as tools for teachers to use for purposes of knowledge building. Specific attention is paid to the role of specialised composition and classification taxonomies and activity sequences in specialised fields, and the relation of this valeur to the concept of semantic density in Legitimation Code Theory. (Contains 14 figures.)