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Note The information on the SI below was current up until May 20, 2019 (World Metrology Day). Since then the SI has undergone major changes. Updated pages will soon become available. In the meantime, users may refer to the Bureau International des Poids et Mesures (BIPM) website. Now that the 9th edition of the SI Brochure (2019) has been made available, we are in the process of revising SP 330, SP 811, and the contents of this site. Essentials of the SI     Introduction     SI units and prefixes     Units outside the SI     Rules and style conventions Background     Definitions of the SI base units and their historical context     International aspects of the SI     Unit conversions Bibliography     Online publications and citations

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