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According to Nyssa's description, an index file was a kind of root directory around which computer systems were organised. When trying to figure out how to access the data stored in the TARDIS information system, she and Tegan were briefly flummoxed by their inability to find the databank's index file. Tegan eventually hit upon the notion that the index file could be recalled by entering the letters I and F into the Information System's interface, which seemed to work. They later learned that some or all of the information they were able to access had been made available to them by the Master. (TV: Castrovalva)