We have long been taught that constitutions are either "written" or "unwritten." But this binary classification is wrong. All constitutions are in some way written, and all constitutions contain unwritten rules. This false distinction moreover overlooks the most important formal difference among the constitutions of the world: some constitutions consist of a single, supreme document of higher law while others consist of multiple documents, each enacted separately with shared supremacy under law. Ubiquitous but so far unnoticed, these constitutions comprising multiple texts are a unique constitutional form that has yet to be studied and theorized. I call them multi-textual constitutions. This Article is the first on multi-textuality as a constitutional form. I draw from current and historical constitutions in Africa, the Americas, Asia, Europe, and Oceania to explain, illustrate, and theorize the design and operation of multi-textual constitutions. I examine their origins, compare how they perform relative to the alternative unitextual constitutional form, and outline a research agenda for further study. What results is a reordering of our basic constitutional categories, a deep analytical dive into a distinct constitutional form, and a disruptive revelation about the United States Constitution, the world's paradigmatic model of a uni-textual constitution.