This article aims at developing an external language of description to investigate the problem of why particular groups of students are systematically not provided access to school mathematical knowledge. Based on Basil Bernstein's conceptualisation of power in classification, we develop a three-dimensional model that operationalises the contextual boundaries, the linguistic features and the structure of knowledge, and their respective relations to power on a not too high level of abstraction. The contribution of this article consists of a systematic connection of classification with the latter two dimensions that abides the generative principles of Bernstein's internal language of description. Finally, we illustrate the analytical potential of the three-dimensional model through a brief discussion of one classroom episode. (Contains 3 figures and 2 notes.)