A clock tower is a tower built with a large clock face on one or more (often all four) of its sides so as to be visible to a large number of inhabitants of an area. The mechanism inside the tower is known as a turret clock . It often marks the hour (and sometimes segments of an hour) by sounding large bells or chimes , sometimes playing simple musical phrases or tunes. The clock tower is usually part of a church or municipal building such as a town hall, but many clock towers are free-standing. Often the tallest structure in town, clock towers were a popular feature of city life before the middle of the twentieth century, when most people didn't have accurate and reliable watches to carry with them. Two of the most well-known clock towers are the Clock Tower, often colloquially referred to as Big Ben, of the Palace of Westminster in London, and the Spasskaya Tower of the Moscow Kremlin.