The Units of measurement reference article from the Simple Wikipedia on 24-Jul-2004
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Units of measurement

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A unit of measurement is an amount used to show how much length, or mass, or volume or other quantities something has. There are many different standards and units used all over the world. One global standard is commonly called the Metric System. It is also known as Système International d'Unités, or SI. The kilometre, metre, centimetre, millimetre are some units of length or linear size in this system. While many countries have officially adopted SI, a wide range of traditional units are still used. For example, in the United States the metric system has been legal for trade since 1866 and is widely used in industry and daily life. Some basic measurements such as the gallon, inch, mile, and the pound are still widely used. See US units of measurement. In Europe, which is largely metric, this system based on tens is widely rearranged into odd, but practical human divisions. For example, the quarter millimetre, and non-power of ten trade quantities which are marked with the EEC or 'e' mark.

In metric:

Standards are very important. Each unit is a set size. A distance or length or volume or mass or span of time being measured is described as a certain number of these units. Sometimes more than one unit is used to get better accuracy, or a smaller unit is used - so one can say "one metre, 95 centimetres" or just "195 centimetres".

The imperial units now used mainly in the United States: inch, foot, yard, mile, and the US gallon, smaller than the old imperial gallon (UK Gallon), about 4.5 litres, which is now not used anywhere much. Some older bottles, jugs and liquid containers are this size. There are 128 fluid ounces in a US gallon, but there are also 16 ounces in a pound, an older measure of weight that is 453.6 grams. These are two different measurements with exactly the same name - it is only when measuring water that they actually mean the same.

A unit of measurement that applies to money is called a unit of account. This is normally a currency issued by a country - and smaller divisions. For instance the US dollar and US cent, which is 1/100 of a dollar. Or the Euro and Eurocent.

Science, medicine and engineering use larger and smaller units of measurement than these, and talk about them more exactly. For instance the difference between mass and weight matters more in these fields.