In the tradition of legal theory, the theoretical account of normative changes, which was offered by Carlos Alchourrón and Eugenio Bulygin, is probably the most influential. One of its distinctive features is the use of various concepts from set theory to explain legal systems. However, previous attempts to determine the role of the essential elements of set theory in explaining legal systems have been insufficient. This article therefore aims to remedy the inadequacy by explaining these elements, providing them with the content of an axiomatic set theory, and showing their role in the theoretical account of normative change in general and repeal, in particular.