John Rawls, "Two Concepts of Rules," Philosophical Review 64 (1955): 3–32.

In this paper I want to show the importance of the distinction between justifying a practice and justifying a particular action falling under it, and I want to explain the logical basis of this distinction and how it is possible to miss its significance. While the distinction has frequently been made, and is now becoming commonplace, there remains the task of explaining the tendency either to overlook it altogether, or to fail to appreciate its importance.

Online:
JSTOR [pdf]
University of California, San Diego [pdf]