Most legislatures have their powers restricted by the nation's Constitution, and Montesquieu's theory of the separation of powers typically restricts a legislature's powers to legislation. Although the legislature has the power to legislate, it is the courts who have the power to interpret statutes, treaties and regulations. Similarly, although parliaments have the power to legislate, it is usually the executive who decides on the legislative programmed. The procedure is usually that a bill is introduced to Parliament, and after the required number of readings, committee stages and amendments, the bill gains approval and becomes an Act.