Guide for the Use of the International System of Units (SI)

Guide for the Use of the International System of Units (SI) 
 
3 
3
Other Sources of Information on the SI 
 
3.1 Publications 
 
Appendix C briefly describes a number of publications that deal with the SI and related topics; 
citations for these publications are given in Appendix D—Bibliography. Additional information about the 
SI is also available from the two NIST organizational units indicated in Secs. 3.2 and 3.3.
3.2 Fundamental Constants Data Center 
 
Questions concerning the more fundamental aspects of the SI and subtle aspects of proper SI usage 
may be directed to:
Fundamental Constants Data Center 
Physics Laboratory 
National Institute of Standards and Technology 
100 Bureau Drive, Stop 8420 
Gaithersburg, MD 20899-8420 
Telephone: [phone] 
Fax: [phone] 
SI Units Web Page: http://physics.nist.gov/cuu/Units/index.html 
3.3 Metric Program 
 
Questions concerning Federal Government use of the SI and Federal Government policy on the use 
of the SI by U.S. industry and the public may be directed to: 
Metric Program 
Technology Services 
National Institute of Standards and Technology 
100 Bureau Drive, Stop 2600 
Gaithersburg, MD 20899-2600 
Telephone: [phone] 
Fax: [phone] 
Email: [e-mail] 
http://nist.gov/metric 
4 The Two Classes of SI Units and the SI Prefixes
 
Since the 1995 edition of this Guide, the 20th CGPM, which met October 9 - 12, 1995, decided to 
eliminate the class of supplementary units as a separate unit class in the SI. The SI now consists of only two 
classes of units: base units and derived units. The radian and steradian, which were the two supplementary 
units, are now subsumed into the class of SI derived units. Thus the SI units are currently divided into base 
units and derived units, which together form what is called “the coherent system of SI units.”
2
The SI also 
includes the prefixes to form decimal multiples and submultiples of SI units. 
2
According to Ref. [4: ISO 31-0], a system of units is coherent with respect to a system of quantities and equations if the system of 
units is chosen in such a way that the equations between numerical values have exactly the same form (including the numerical 
factors) as the corresponding equations between the quantities (see Secs. 7.11 and 7.14). In such a coherent system, of which the SI is 
an example, no numerical factor other than the number 1 ever occurs in the expressions for the derived units in terms of the base units.

---