In the context of CSCW ? especially through ethnomethodological work place studies - the stability of particular work practices and therefore the ability to design software that fits with continually evolving work practices is questioned. This challenge for software development has been called 'design for unanticipated use'. Using the concept of interpretability, I attempt to answer this challenge. A semiotic perspective on computer applications as formal symbol manipulation systems is introduced. A case study involving three alternative ways of using a computer application shows how users make sense of such symbolic machines. Wittgenstein's concept of language games is used as a 'figure of thought' to relate practice, language, and the use of symbolic machines. The development of an interpretation, fitting the implemented symbol manipulation and supporting the specific understanding of the task, remains crucial for competent use. Interpretability is introduced as a quality of computer applications. In order how to support the user in developing her own interpretation, a concept for help systems is described.