A type of shelf clock is the crystal regulator clock, dating from the 1800s. This style is surrounded on all sides with glass, thus showing the interior movements. Original the crystal regulators were extremely accurate with a pendulum consisting of two small glass vials filled with mercury. However, with their rise in popularity, came cheaper, less accurate imitations where the pendulums were merely polished metal cylinders. The 'Anniversary' trademark on 400-day clocks was introduced in 1901.