An hour is a unit of time historically reckoned as 1⁄24 of a day and defined contemporarily as exactly 3,600 seconds (SI). There are 60 minutes in an hour, and 24 hours in a day.
A 7th-century Saxon tide dial on the porch at Bishopstone in Sussex, with larger crosses marking the canonical hours.
Sundial with Italian hours in Asti
A Chinese diagram from Su Song's AD 1092 Xinyi Xiangfa Yao illustrating his clocktower at Kaifeng.
A reconstruction of another kind of Chinese clepsydra in Beijing's Drum Tower
The metric system is a decimal-based system of measurement. The current international standard for the metric system is the International System of Units, in which all units can be expressed in terms of seven base units: the metre, kilogram, second, ampere, kelvin, mole, and candela.
A kilogram mass and three metric measuring devices: a tape measure in centimetres, a thermometer in degrees Celsius, and a multimeter that measures potential in volts, current in amperes and resistance in ohms.
Pavillon de Breteuil, Saint-Cloud, France, the home of the metric system since 1875
James Clerk Maxwell played a major role in developing the concept of a coherent CGS system and in extending the metric system to include electrical units.