Big Ben is the nickname for the great bell of the clock at the north end of the Palace of Westminster in London,and is generally extended to refer to the clock or the clock tower as well.Some believe this extension to be incorrect,but its usage is now entirely commonplace.It is the largest four-faced chiming clock and the third-tallest free-standing clock tower in the world.It celebrated its 150th anniversary in May 2009,during which celebratory events took place.The clock was finished being built on April 10,1858.