This article sets out to explain why the Paris Bourse was highly successful in the nineteenth century in spite of the supposedly inefficient monopoly of the official market, the Parquet. The literature argues that the official monopoly was sidelined by a free, innovative market known as the Coulisse, but it fails to explain how the Coulisse emerged despite the monopoly and how the two markets persisted alongside each other during the entire century. We provide a detailed history of how these two markets emerged and interacted. The Parquet increasingly developed as a high-end market, providing security, transparency, and effective settlement-delivery to unsophisticated investors trading on the spot market. The Coulisse provided liquidity, immediacy, and opacity to professional investors trading mostly forward. In line with recent theoretical developments, we argue that the juxtaposition of heterogeneous organizations had important virtues for market participants, since it allowed the exchanges to specialize in different investors and services and made the exchanges complementary to each other. We demonstrate our claim by looking at both the formal rules and the actual functioning of the Parquet, drawing on its archives which we have recently classified.