A written plan of government is called a constitution, and it lays out the way a state will be structured through government organizations while also formalizing the various roles and powers. Most countries have a form of constitution, although they are not always written documents.
The United States Constitution is perhaps one of the most well known examples in the world and was signed into law in 1787 in Pennsylvania. Like many other constitutions, the American constitution is not set in stone. It is open to interpretation and ratification from the Supreme Court, usually as a result of cases which required a change in constitutional law.
Many countries do not have a single document that outlines the constitution, with some having no written documents at all. The U.K. constitution, for example, is mainly based on tradition and a number of ongoing conventions, with no single unifying document.
While changing a constitution, especially a written one, can be a very difficult process, the legal branch of government is often responsible for making any changes. The enforcement of most constitutions is dependent on how they are interpreted by law. This is the same for written and nonwritten constitutions.