Usually found on an upper extension of a structure, a tower clock consists of one, two, three, or four clocks. Tower clocks are frequently found on buildings, but they can also be found on free-standing towers or other structures, such cell phone antennae towers. Tower clocks are big, outdoor clocks that can be illuminated or not. Either independent or central clock movements can operate the clocks. Modern, maintenance-free electric clock movements and the electronic controls that go with them have replaced the once-common mechanical tower clock movements.