Both displayed (public) and hidden (private) liquidity play important roles in market structure. Displayed liquidity allows prices to converge. When market makers compete, a reasonable matching engine will ensure that users get filled at similar, fair prices. Public information is then emitted through the public orderbook, trade list, or both. Hidden liquidity can emerge in 2 ways: 1. A market for Widgets shows a bid at 100 and an offer at 101. Alice places a limit order to buy at 100.60 and immediately gets filled. There was hidden liquidity to sell at 100.60, but no publicly displayed limit order. 2. Bob asks for a quote to trade 100 Foos for Bars (RFQ). He can sell 100 Foos for $1000 and use that $1000 to buy 499 Bars on the open market. This implies an exchange rate of 4.99, post transaction fees. Instead, he receives a quote of 499.5 Bars for 100 Foos. Because there is no public Foo-Bar market, the market maker who filled his request put no capital at risk. However, the hidden liquidity for this asset pair allowed Bob to receive a more favorable exchange rate. In a well-functioning market, public price information ensures that users get competitive prices on liquid, public trading pairs. The public signals then enable price improvement for hidden liquidity. There is clearly a role for both; there's an open problem of which structures facilitate the most positive-sum activity while leaking the least amount of extractive value. Abstractly, this reduces to a graph where the nodes are assets and the edges represent different asset markets. Global liquidity (displayed + hidden) is a measurement of how well-connected the graph is.