Abstract: A funny thing about the U.S. Constitution is that it’s written down. Words might seem like an obvious feature of a constitution, but they're notably missing from much of the constitution of the United Kingdom, the country from which the United States seceded. Historians have often assumed that the quirky American practice of putting constitutions into single documents has its origins in the corporate charters of the seventeenth-century trading companies that founded more than half of the thirteen original states. But, as historian Mary Bilder has written, it is surprisingly difficult to explain the change from corporate charter to modern constitution with precision and persuasive power. This Article attempts to do just that, telling the story of an eighty-year lawsuit that forced the Massachusetts Bay Company to treat its charter's terms as Gospel. Relying on original research of thousands of primary sources from the United States and United Kingdom spanning from 1607 through 1793, the Article presents an account of how a corporate charter evolved into a “Charter Constitution” in America while the British Constitution remained intangible. The Article demonstrates that written words became a defining feature of American constitutionalism a century before the American Revolution, and that this distinction between the American and British understanding of constitutions contributed to American independence. It also demonstrates that charter constitutionalism emphasized text but also included methods of interpretation that today might be described as purposivist or living constitutionalist.