Clocks can be classified by the type of time display, as well as by the method of timekeeping.
Time display methods
A linear clock at London's Piccadilly Circus tube station
The 24 hour band moves across the static map, keeping pace with the apparent movement of the sun above ground, and a pointer fixed on London points to the current time
Analog clocks usually indicate time using angles
The most common clock face uses a fixed numbered dial or dials and moving hand or hands
It usually has a circular scale of 12 hours, which can also serve as a scale of 60 minutes, and 60 seconds if the clock has a second hand
Many other styles and designs have been used throughout the years, including dials divided into 6, 8, 10, and 24 hours
The only other widely used clock face today is the 24 hour analog dial, because of the use of 24 hour time in military organizations and timetables
The 10-hour clock was briefly popular during the French Revolution, when the metric system was applied to time measurement, and an Italian 6 hour clock was developed in the 18th century, presumably to save power (a clock or watch striking 24 times uses more power).
Another type of analog clock is the sundial, which tracks the sun continuously, registering the time by the shadow position of its gnomon
Sundials use some or part of the 24 hour analog dial
There also exist clocks which use a digital display despite having an analog mechanism—these are commonly referred to as flip clocks.
Alternative systems have been proposed
For example, the Twelve o'clock indicates the current hour using one of twelve colors, and indicates the minute by showing a proportion of a circular disk, similar to a moon phase.
Digital clock outside Kanazawa Station displaying the time by controlling valves on a fountain
Digital clocks display a numeric representation of time
Two numeric display formats are commonly used on digital clocks:
the 24-hour notation with hours ranging 00–23;