How can firms profitably give away free products? This paper provides a novel answer and articulates trade-offs in a space of information product design. We introduce a formal model of two-sided network externalities based in textbook economics---a mix of Katz and Shapiro network effects, price discrimination, and product differentiation. Externality-based complements, however, exploit a different mechanism than either tying or lock-in even as they help to explain many recent strategies such as those of firms selling operating systems, Internet browsers, games, music, and video. The model presented here argues for three simple but useful results. First, even in the absence of competition, a firm can rationally invest in a product it intends to give away into perpetuity. Second, we identify distinct markets for content providers and end consumers and show that either can be a candidate for a free good. Third, product coupling across markets can increase consumer welfare even as it increases firm profits. The model also generates testable hypotheses on the size and direction of network effects while offering insights to regulators seeking to apply antitrust law to network markets.network effects, network externalities, two-sided markets, free information, business models, strategic complements, product design

Due to network effects and switching costs in platform markets, entrants generally must offer revolutionary functionality to win substantial market share. We explore a second entry path that does not rely upon Schumpeterian innovation: platform envelopment. Through envelopment, a provider in one platform market can enter another platform market, and combine its own functionality with that of the target in a multi-platform bundle that leverages shared user relationships. Envelopers capture market share by foreclosing an incumbent's access to users; in doing so, they harness the network effects that previously had protected the incumbent. We present a typology of envelopment attacks based on whether platform pairs are complements, weak substitutes, or functionally unrelated and we analyze conditions under which these attack types are likely to succeed.

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