The ‘graphicacy’ of student programmers was investigated using several cognitive tasks designed to assess ER knowledge representation at the perceptual, semantic and output levels of the cognitive system. A large corpus of external representations (ERs) was used as stimuli. The question ‘How domain-specific is the ER knowledge of programmers?’ was addressed. Results showed that performance for programming-specific ER forms was equal to or slightly better than performance for nonspecific ERs on the decision, naming and functional knowledge tasks, but not the categorisation task. Surprisingly, tree and network diagrams were particularly poorly named and categorised. Across the ER tasks, performance was found to be highest for textual ERs, lists, maps and notations (more ubiquitous, ‘everyday’ ER forms). Decision task performance was generally good across ER types indicating that participants were able to recognise the visual form of a wide range of ERs at a perceptual leve...