Big Ben is the nickname for the great bell of the clock at the north end of the Palace of Westminster in London, and often extended to refer to the clock and the clock tower, officially named Elizabeth Tower, as well. Elizabeth Tower holds the largest four-faced chiming clock in the world and is the third-tallest free-standing clock tower. It celebrated its 150th anniversary on 31 May 2009, during which celebratory events took place. The tower was completed in 1858 and has become one of the most prominent symbols of both London and England, often in the establishing shot of films set in the city.
The Elizabeth Tower (previously called the Clock Tower) named in tribute to Queen Elizabeth II in her Diamond Jubilee year – was raised as a part of Charles Barry's design for a new palace, after the old Palace of Westminster was largely destroyed by fire on the night of 16 October 1834. The new Parliament was built in a Neo-gothic style. Although Barry was the chief architect of the Palace, he turned to Augustus Pugin for the design of the clock tower, which resembles earlier Pugin designs, including one for Scarisbrick Hall. The design for the Elizabeth Tower was Pugin's last design before his final descent into madness and death, and Pugin himself wrote, at the time of Barry's last visit to him to collect the drawings: "I never worked so hard in my life for Mr Barry for tomorrow I render all the designs for finishing his bell tower & it is beautiful." The tower is designed in Pugin's celebrated Gothic Revival style, and is 315 feet (96.0 m) high (roughly 16 storeys).
The bottom 200 feet (61.0 m) of Elizabeth Tower's structure consists of brickwork with sand coloured Anston limestone cladding. The remainder of the tower's height is a framed spire of cast iron. The tower is founded on a 50 feet (15.2 m) square raft, made of 10 feet (3.0 m) thick concrete, at a depth of 13 feet (4.0 m) below ground level. The four clock dials are 180 feet (54.9 m) above ground. The interior volume of the tower is 164,200 cubic feet (4,650 cubic metres).
Despite being one of the world's most famous tourist attractions, the interior of the tower is not open to overseas visitors, though United Kingdom residents are able to arrange tours (well in advance) through their Member of Parliament. However, the tower has no lift, so those escorted must climb the 334 limestone stairs to the top.
Due to changes in ground conditions since construction, the tower leans slightly to the north-west, by roughly 230 millimetres (9.1 in) over 55 m height, giving an inclination of approximately 1/240. This includes a planned maximum of 22 mm increased tilt due to tunnelling for the Jubilee Line extension) Due to thermal effects it oscillates annually by a few millimetres east and west.