SI plays an essential role in international commerce and is the common language of scientific and technological research and development. The 2008 edition of NIST's "The International System of Units (SI)" (Special Publication 330) covers recent changes in the system since 1998. It "Americanizes" the SI to cover correct U.S. usage of metric units, such as the accepted spelling of "meter" and "liter." In addition, the revised guide includes a new chapter on units for quantities that describe biological effects, and symbols for expressing values for enzyme catalytic activity in biology and medicine.
The U.S. Omnibus Trade and Competitiveness Act of 1988 designated "the metric system of measurement as the preferred system of weights and measures for United States trade and commerce." U.S. Public Law 110-69 in 2007, also known as the "American Competes Act," replaced the old definition of the metric system with the definition of the SI.
The 2008 editions of "The International System of Units", edited by Barry N. Taylor and Ambler Thompson, is available online along with additional information and measurement conversion tables at Technology Services: Metric Information and Conversions. Hard copies can be requested by e-mail at TheSI@nist.gov.