The article deals with the constitutional and legal basis for determining the system of parliamentary law: The subject matter, methods, and sources. It is stated that legal norms characterized by common features, internal unity and different from the norms of other branches of law form an independent branch (a sub-branch of constitutional) law. For now, the institutions of parliamentary law are studied within the framework of the theory of state and law, constitutional (state) law, and, to some extent, administrative law, and parliamentary procedural law is distinguished. In this regard, the article notes that the development of democracy and parliamentarism, the increasing importance of Parliament in the implementation of the principle of separation of powers always compels the legal doctrine to separate parliamentary law. The author substantiates the conclusion that constitutional law, as the leading branch of public law, regulates social relations that are usually called basic (constitutional) — or fundamental — in each area of life, while parliamentary law has theoretical and legal prerequisites for being separated into an independent branch (sub-branch of constitutional) law without violating the organic unity of constitutional law. Thus, the author believes that at the present level of democracy development it is possible to state the presence of prerequisites for the formation of a new branch (sub — branch) of law — parliamentary law.