Clock Tower, Palace of Westminster

The Clock Tower is a turret clock structure at the north-eastern end of the Houses of Parliament building in Westminster, London. It is colloquially and popularly known as Big Ben, however this name actually belongs to the clock's main bell. It has also been referred to as St. Stephen's Tower.

The Clock Faces were once large enough to allow the Clock Tower to be the largest four-faced clock in the world, but has since been outdone by the Allen-Bradley Clock Tower in Milwaukee, Wisconsin. The builders of the Allen-Bradley Clock Tower did not add chimes to the clock, so Big Ben still holds the title of the ‘World's largest four-faced chiming clock’. The clock mechanism itself was completed by 1854, but the tower was not fully constructed until four years later in 1858.

Big Ben, officially known as the Great Bell of Westminster, is the largest bell in the tower and part of the Great Clock of Westminster. The Great Bell weighs 13 metric tonnes and is 2.2 metres high.

The exact origin of the name 'Big Ben' has remained a popular mystery, leading to speculation that suggests the bell was named after heavyweight boxer Benjamin Caunt who was popular at the time, however an alternate theory that has been supported cites the origin of the name as belonging to Sir Benjamin Hall who was the Parliamentary Commissioner of Works.

The clock is famous for its reliability. This is due to the skill of its designer, the lawyer and amateur horologist Edmund Beckett Denison. Denison invented the double three-legged gravity escapement. This escapement provides the best separation between pendulum and clock mechanism. Together with an enclosed, wind-proof box sunk beneath the clockroom, the Great Clock's pendulum is well isolated from external factors like snow, ice and pigeons on the clock hands, and keeps remarkably accurate time.

Big Ben is a focus of New Year celebrations in the United Kingdom, with radio and TV stations tuning to its chimes to welcome the start of the year. Similarly, on Remembrance Day, the chimes of Big Ben are broadcast to mark the 11th hour of the 11th day of the 11th month and the start of two minutes' silence.