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Bipm: The International System of Units (Si)
BIPM (Bureau International des Poids et Mesures) is an international organization, located in France, an equivalent of our National Institute of Standards and Technology (NIST). This site is the BIPM intro page to their official...
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Bipm: Si Brochure
Here's where to obtain your official brochure on SI, from the people who derived it. Little info at this site, except how to order an official brochure (and its cost). Or you can download a copy free.
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Wfp Consulting: Si Navigator
A commercial site which has links to many SI/metrics sites. It gets you to a host of sites. Don't hesitate to use it for research.
National Institute of Standards and Technology (NIST)
International Systemn of Units (Si)
The official sources from which NIST forms its decisions for "official usage". Most useful if you're doing serious research about SI.