The oldest part of the Orloj, the mechanical clock and astronomical dial, dates from 1410 when it was made by clockmaker Mikuláš of Kadaň and Jan Šindel, then later a professor of mathematics and astronomy at Charles University.
The Orloj is mounted on the southern wall of Old Town Hall in the Old Town Square. The clock mechanism has the astronomical dial, representing the position of the Sun and Moon in the sky and displaying various astronomical details. Statues of various Catholic saints stand on either side of the clock, an hourly show of moving Apostle figures and other sculptures, notably a figure of a skeleton that represents Death, striking the time and a calendar dial with medallions representing the months. According to local legend, the city will suffer if the clock is neglected.