We investigate competition between stock exchanges that choose the number of trading platforms to establish and the fee structure on each platform. U.S. exchanges compete for order flow by setting “make” fees for limit orders and “take” fees for market orders. When traders can quote continuous prices, the manner in which exchanges divide the total fee between makers and takers is irrelevant, because traders can choose prices that perfectly counteract any division of the fee. In such a case, order flow will simply consolidate to the platform with the lowest total fee. The one-cent minimum tick size constraints imposed by SEC Rule 612 (Minimum Pricing Increment) to traders prevent perfect neutralization and also destroy mutually agreeable trades at price levels that range within a tick. These frictions create both scope and incentive for an exchange to establish multiple platforms with different fee structures in order to engage in second-degree price discrimination, and lead to mixed-strategy equilibria with positive profits for intrinsically identical competing platforms, rather than to zero-fee, zero-profit Bertrand equilibrium. We show that price discrimination via platforms with differing fee structures can Pareto-improve social welfare in the presence of tick-size constraints. These theoretical results rationalize the diversity of fee structures, the frequent fee adjustments, and the proliferation of stock exchanges that have been observed empirically.