2-Measurement Fundamentals - 2: Systems of Measurement
There are two systems of measurement used in machine shops today, the Metric system and the Inch system.

The Metric system is based on the meter as the standard unit of reference. A meter (approximately 39.37 inches in length) is subdivided into 10 equal parts called decimeters. Each decimeter is divided into 10 parts called centimeters and each centimeter is divided into 10 parts called millimeters.

The Metric system is a very coherent system because it is exclusively a decimal system and therefore has a common multiplier and divisor of 10. Regular fractions are not used in the metric system. Instead the metric system uses only decimal fractions. Other names for the Metric system include SI units and ISO units. The Metric system is the most commonly used system of measurement in the world.

The Inch system is based on the foot as the standard unit of reference. A foot is divided into 12 equal parts called inches. Each inch is subdivided into a variety of fractions and decimals.

The Inch System is a rather incoherent system because it lacks the decimal based advantage of the Metric System. Parts of a foot can not be easily expressed as decimal inches. For example, in the metric system 7 millimeters is 0.7 centimeters which is 0.07 decimeters which is 0.007 meters. But 7 inches is 0.583333 feet which is 0.19444 yards and so on. This is a clear advantage for the metric system.

Another name for the Inch System is the English System. The Inch System is used in most English speaking countries but is being slowly replaced by the metric system.