This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Vatican City is the smallest sovereign state in the world, and almost uniquely among religious legal systems, canon law is not just a source of its law. It is the primary one. The Fundamental Law of Vatican City State, the document that organizes the state's own courts and offices, names canon law as the chief body of law the state runs on, with the state's own civil legislation filling in only what canon law does not reach.
That arrangement makes Vatican City the one place on earth where a body of religious law is also, directly, the positive law of a recognized state, the way common law is the law of England or civil law is the law of France. Everywhere else the Catholic Church's own legal system, codified today in the 1983 Code of Canon Law for the Latin Church and the 1990 Code of Canons of the Eastern Churches, functions purely as the Church's own internal law: it binds clergy, religious institutions and the practice of the sacraments, but it has no force in the civil law of France, Italy, the United States or any other state whose courts do not choose to enforce it.
The Church runs its own court system to apply that law, a hierarchy of diocesan tribunals with, on appeal, the Roman Rota in Vatican City, a tribunal that traces informally to the twelfth century and took its settled form under a papal bull in 1331. Most of what reaches the Rota concerns whether a marriage was validly formed under canon law, the question a Catholic seeking to remarry within the Church must have answered, but the tribunal's competence reaches any canon-law matter referred to it. It is, in other words, an appellate court doing exactly the work an appellate court does anywhere else. What makes it unusual is only the law it applies, and the fact that in one small state, that law is also the law of the land.