To better understand how external representations support learning and doing physics, we present a theoretical analysis of representations informed by perspectives from cognitive science, including information processing and distributed cognition. We describe external representations in terms of three aspects: organization of information about a conceptual referent presented in a medium. To provide a more detailed description that is more closely linked to instruction and learning, we consider the intersection of the form and function of representations and identify a set of nine struc- tural features. For example, literalness refers to how literally the representation captures the thing being represented. We illustrate these structural features with four examples where we compare and contrast the characteristics of different representations. We argue that learners and professionals often benefit from different structural features and discuss ways of sequencing representations for learners.