Cognitive actors cannot coordinate through knowledge alone—they require shared belief systems that align expectations and communication channels that maintain alignment. Large-scale cooperation is rooted in common myths—fictions that exist only in collective imagination yet enable millions to work toward shared goals. At smaller scales, common ground reduces coordination overhead through shared presuppositions and commitments. Conventions emerge as solutions to coordination problems—stable equilibria where expectations align. Common knowledge forces convergence: agents with shared priors who learn each other’s conclusions cannot rationally disagree. Communication channels maintain coherence as conditions change; channel richness determines capacity to reduce ambiguity and sustain shared understanding. Prices coordinate dispersed knowledge through markets; institutions structure interaction to reduce uncertainty. Organizations, markets, and institutions are all mechanisms for sustaining these shared structures at scale.