Although constitutions establish countries’ fundamental laws, identifying which legal materials are constitutional is not a straightforward task. Empirical studies in comparative law have focused on countries’ formal, written constitutions—the “large-C” constitutions—while often ignoring the broader body of constitutional rules—the “small-c” constitutions. This oversight occurs because incorporating small-c constitutional law into comparative research requires overcoming the conceptual challenge of identifying which legal sources should be considered constitutional and the practical challenge of quantifying those sources across jurisdictions. In this article, we address both of these challenges. We address the conceptual challenge by outlining three approaches to identifying small-c constitutions: an entrenchment-based approach that emphasizes whether given rules are more deeply entrenched than conventional laws, an external approach that emphasizes exogenously defined core constitutional functions, and an internal approach that emphasizes local understandings of constitutional law. We then partially address the practical challenge by using an internal approach to collect data on one aspect of constitutional law: constitutional rights. We collected this data for 123 countries by conducting an expert survey of constitutional law scholars and practitioners. Our data reveals that although the large-C constitution is the primary source of constitutional rights in a majority of countries, the small-c constitution is still a notable source of rights, especially in older constitutional systems. But although our data offers new evidence about small-c constitutional rights around the world, it also points to several substantial obstacles that any efforts to code small-c constitutions are likely to face.