The United Kingdom is a unitary state under a constitutional monarchy. Queen Elizabeth II is head of state of the UK as well as of fifteen other independent Commonwealth countries. The United Kingdom has an uncodified constitution, as do only three other countries in the world. The Constitution of the United Kingdom thus consists mostly of a collection of disparate written sources, including statutes, judge-made case law, and international treaties, together with constitutional conventions. As there is no technical difference between ordinary statutes and "constitutional law," the UK Parliament can perform "constitutional reform" simply by passing Acts of Parliament and thus has the political power to change or abolish almost any written or unwritten element of the constitution. However, no Parliament can pass laws that future Parliaments cannot change.