This article is one of a series devoted to the study of a priori in law. It is part of the ANTECEDENT research project supported by a Chair of the Initiative of Excellence Université Côte d'Azur sponsored by the French government. The aim of the project is to revisit the law through its preunderstandings, leave behind the law only better to return to it, propose an "antecedent" theory of law and apply theoretical reflection to major topical issues. It is based on the following general hypothesis: if a priori are everywhere, and if the law is capable of dealing with a priori, then the law is everywhere, and not just where we want to leave it. After studying the antecedent method and the vocabulary of a priori, we devote this third article to examining the grammar of a priori in and around the law. Contents 1. The grammar of a priori in the law 2. The grammar of a priori outside of law 3. The foundations of a comprehensive grammar of a priori in law