| 21. | Historically the " foot " was a part of many local systems of units, including the digits.
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| 22. | The manual then describes a system of unit identification that sounds like a fore-runner of medieval heraldry.
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| 23. | The base unit of energy within the International System of Units ( SI ) is the joule.
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| 24. | A standard system of units facilitates this.
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| 25. | SI is the official system of units, but traditional units are still ubiquitously used in everyday life.
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| 26. | Metric systems of units have evolved since the adoption of the first well-defined system in France in 1795.
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| 27. | The standard International System of Units ( SI ) unit of mass is the kilogram ( kg ).
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| 28. | Despite its name, IU is not part of the International System of Units used in physics and chemistry.
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| 29. | Finally, one more unit is needed to construct a usable system of units that includes energy and mass.
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| 30. | The standards-based unit in the International System of Units ( SI ) is the joule ( J ).
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