Guaranteeing Consistency in Text-Based Human-Computer Interaction Bernhard Beckert, Gerd Beuster Wrong assumptions about the state of the computer system are a main source of error in human-computer interaction. We show how consistency requirements between the state of a computer system and the user's assumptions about the state can be formally defined. The definition of HCI consistency is used to show correctness of a methodology to ensure consistency for TTY-based applications.