National legal systems are not isolated from foreign influences. They are part of a “transnational legal space” where they participate in an intensely hierarchical dialogue of experiences between legal systems. Modern Comparative Law was born at the dawn of the 20th Century in a plateau of intense cosmopolitism that impacted too the geographical imagination and mental maps of lawyers and of the legal discipline at large. The article aims at presenting and contextualizing el foundational moment of modern comparative law. To do this, it puts forward a theoretical interpretation of the methodological and substantive discussions carried out in the World Congress of Comparative Law of Paris, 1900 (under the direction of French scholars Raymond Saleilles and Édouard Lambert). The article defends the proposition that, around that time, a “transnational legal space” was being consolidated where national sovereignties opened up their national legal systems in a dynamic process of influence by comparative and foreign legal materials. Latin American countries, together with others that were undergoing processes of intense modernization (like Turkey and Japan), undertook enthusiastically their own participation in the transnational legal space. Around the same time the basic modern methods of the discipline were being stabilized, and many points of them continue to be dominant nowadays. The article, finally, pretends to explain to lawyers the basic constitutive geography of modern comparative law and its methodological protocols, with a particular interest in the position of interest of Latin American legal systems in the foundation of Comparative Law as both space and discipline.