In the United States the three branches of government are completely separate except for the Vice President who is President of the Senate. In the United Kingdom the three branches of Government are mixed but the checks and balances are provided by history and "custom" (the rule that says something should happen because that is how it has been done for a long time). The Queen is Head of State (the executive), but is also part of Parliament (the legislative branch) and is the Fountain of Justice (the head of the judicial branch). But by convention she does not do anything without the advice of Ministers and never refuses to pass an Act of Parliament. The Queen has a lot of power but the power is controlled and balanced by the need to act in certain ways or only use the power at certain times.