The article reveals the legal nature of the set of rules of law. It is noted that the first mentions of legal institutions are found in Guy's "Institution". Various approaches to understanding the set of rules of law are considered. A comparative analysis of the set of rules of law and the branch of law has been carried out. The concept of a sub-institution is proposed and its features are disclosed. The legal nature of intersectoral institutions has been studied. It is concluded that the set of rules of law can be considered not only as an element of the system of law, but also from the standpoint of the sociology of law. With this approach, legal institutions are called social and legal institutions, which act as institutional components of the legal sphere of society.