The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom or Britain, is a sovereign country located off the northwestern coast of the European mainland. The United Kingdom includes the island of Great Britain, the northeastern part of the island of Ireland, and many smaller islands. Northern Ireland shares a land border with the Republic of Ireland. Otherwise, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the southwest, giving it the 12th-longest coastline in the world. The Irish Sea separates Great Britain and Ireland. The total area of the United Kingdom is 94,000 square miles . The United Kingdom is a unitary parliamentary democracy and constitutional monarchy. The monarch is Queen Elizabeth II, who has reigned since 1952, making her the world's longest-serving current head of state. The United Kingdom's capital is London, a global city and financial centre with an urban area population of 10.3 million. The United Kingdom consists of four countries: England, Scotland, Wales and Northern Ireland. Their capitals are London, Edinburgh, Cardiff and Belfast, respectively. Apart from England, the countries have their own devolved governments, each with varying powers. Other major cities include Birmingham, Glasgow, Leeds, Liverpool, and Manchester. The nearby Isle of Man, Bailiwick of Guernsey and Bailiwick of Jersey are not part of the UK, being Crown dependencies with the British Government responsible for defence and international representation. The union between the Kingdom of England and the Kingdom of Scotland in 1707 to form the Kingdom of Great Britain, followed by the union in 1801 of Great Britain with the Kingdom of Ireland created the United Kingdom of Great Britain and Ireland. Five-sixths of Ireland seceded from the UK in 1922, leaving the present formulation of the United Kingdom of Great Britain and Northern Ireland. The UK's name was adopted in 1927 to reflect the change. There are fourteen British Overseas Territories, the remnants of the British Empire which, at its height in the 1920s, encompassed almost a quarter of the world's landmass and was the largest empire in history. British influence can be observed in the language, culture and political systems of many of its former colonies. The United Kingdom has the world's sixth-largest economy by nominal gross domestic product , and the ninth-largest by purchasing power parity . It has a high-income economy and a very high human development index rating, ranking 15th in the world. It was the world's first industrialised country and the world's foremost power during the 19th and early 20th centuries. The UK remains a great power, with considerable economic, cultural, military, scientific and political influence internationally. It is a recognised nuclear weapons state and is sixth in military expenditure in the world. It has been a permanent member of the United Nations Security Council since its first session in 1946. The United Kingdom is a leading member of the Commonwealth of Nations, the Council of Europe, the G7, the G20, NATO, the Organisation for Economic Co-operation and Development , Interpol and the World Trade Organization . It was a member of the European Union and its predecessor, the European Economic Community from 1 January 1973 until withdrawing on 31 January 2020.