Pre-print, Working paper: Theoretical and historical experience suggests a financial centre may either include a single, consolidated and loosely regulated stock exchange attracting all intermediaries and actors, or a variety of exchanges going from strictly regulated to completely unregulated and adapted to the needs of different categories of intermediaries, investors and issuers. Choosing between these two solutions is uneasy because few substantial changes occur at this "meta-regulatory" level. The history of the Paris exchanges provides a good example, since two legal changes in opposite directions occurred in the late 19th century, when Paris was the second financial centre in the world. In 1893, a law threatened the existing two-exchanges equilibrium by diminishing the advantages of the more regulated exchange; in 1898, another law brought them back. We analyse the impact of these two changes on the competition between the exchanges in terms of securities listed, traded volumes and spreads. We conclude competition among exchanges is a delicate matter and efficiency is not always where one would think.