|  |  | The Metric System

The Système International d'Unités (SI), the modern form of the metric system, is the most widely used system of units and measures around the world. The meter (m), which is approximately 39.37 inches, was originally defined as one ten-millionth of the distance from the equator to the North Pole. It was later redefined in terms of the wavelengths of red light from a krypton-86 source. Most recently the meter was defined as the length of the path traveled by light in a vacuum during 1/299,792,458 of a second.

The metric system is based on 10s. For example, 10 decimeters make a meter (39.37 inches).

Units smaller than a meter have Latin prefixes:

- Deci- means 1/10; 10 decimeters make a meter.
- Centi- means 1/100; 100 centimeters make a meter.
- Milli- means 1/1,000; 1,000 millimeters make a meter.

Units larger than a meter have Greek prefixes:

- Hecto- means 100; a hectometer is 100 meters.
- Kilo- means 1,000; a kilometer is 1,000 meters.

A meter is a little more than a yard. A kilometer is less than a mile.

Unit | Value
--- | ---
Kilometer (km) | 1,000 Meters
Hectometer (hm) | 100 Meters
Meter (m) | 1 Meter
Decimeter (dm) | 1/10 Meter
Centimeter (cm) | 1/100 Meters
Millimeter (mm) | 1/1,000 Meters

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