This paper traces the history of TEX, the open source typesetting program. TEX was an early and very successful open source project that imposed its standards in a particularly competitive environment and inspired many advances in the typesetting industry. Developed over three decades, TEX came into competition with a variety of open source and proprietary alternatives. I argue from this case study that open source developers derive direct and indirect network externalities from the use of their software by others and must therefore consider non-developers' needs to make their software more attractive to a broader audience and more competitive with proprietary alternatives.
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