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They are grouped by orders of magnitude, and each section covers three orders of magnitude, or a factor of one thousand. More at Wikipedia
International System of Units (SI) - The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten.
SI prefix - The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes.
 
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