Measurements most commonly use the International System of Units (SI) as a comparison framework . The system defines seven fundamental units : kilogram , metre , candela , second , ampere , kelvin , and mo le . Six of these units are defined without reference to a particular physical object which serves as a standard (artifact-free ), while the kilogram is still embodied in an artifact which rests at the headquarters of the International Bureau of Weights and Measures in Sèvres near Paris . Artifact-free definitions fix measurements at an exact value related to a physical constant or other invariable phenomena in nature, in contrast to standard artifacts which are subject to deterioration or destruction . Instead, the measurement unit can only ever change through increased accuracy in determining the value of the constant it is tied to.
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