The mechanical anniversary clock, also known as the 400-day clock or torsion pendulum clock, is one of the most popular clocks ever invented. Anniversary clocks are recognizable by the decorative balls beneath the dial that rotate one way and then the other. To house the attractive mechanism, every style of clock case has been used although perhaps the most endearing is the glass dome. The dials, hands and ornamentation have also varied steadily over the last century. Called the anniversary clock because it needed winding only once a year, it has carried on into the days of no winding at all.