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International aspects of the SI

The International System of Units, universally abbreviated SI (from the French Le Système International d'Unités), is the modern metric system of measurement. The SI was established in 1960 by the 11th General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures). The CGPM is the international authority that ensures wide dissemination of the SI and modifies the SI as necessary to reflect the latest advances in science and technology.

The CGPM is an intergovernmental treaty organization created by a diplomatic treaty called the Meter Convention (Convention du Mètre, often called the Treaty of the Meter in the United States). The Meter Convention was signed in Paris in 1875 by representatives of seventeen nations, including the United States. There are now 51 Member States of the Meter Convention, including all the major industrialized countries. The Convention, modified slightly in 1921, remains the basis of all international agreement on units of measurement.

The Meter Convention also created the International Bureau of Weights and Measures (BIPM, Bureau International des Poids et Mesures) and the International Committee for Weights and Measures (CIPM, Comité International des Poids et Mesures). The BIPM, which is located in Sèvres, a suburb of Paris, France, and which has the task of ensuring worldwide unification of physical measurements, operates under the exclusive supervision of the CIPM, which itself comes under the authority of the CGPM. 

The CGPM consists of delegates from all the Member States of the Meter Convention and currently meets every four years (the 23nd CGPM took place in November, 2003). The CIPM consists of eighteen members, each belonging to a different Member State; it currently meets every year, usually in September or October at the BIPM. 

Suggested modifications to the SI are submitted to the CGPM by the CIPM for formal adoption. The CIPM may also on its own authority pass clarifying resolutions and recommendations regarding the SI (these resolutions and recommendations usually deal with matters of interpretation and usage). 

To assist it in its broad spectrum of technical activities, the CIPM has set up a number of Consultative Committees (Comités Consultatifs). These committees provide the CIPM with information on matters that it refers to them for study and advice. Each Consultative Committee, the Chairman of which is normally a member of the CIPM, is composed of delegates from national metrology institutes such as NIST, specialized institutes, and other international organizations, as well as individual members. The Consultative Committee for Units (CCU, Comité Consultatif des Unités), which was set up in 1964 and which replaced the Commission for the System of Units set up by the CIPM in 1954, advises the CIPM on matters dealing with the SI. In particular, the CCU helps to draft the BIPM SI Brochure, of which NIST Special Publication 330 (SP 330) is the U.S. version. Indeed, the 8th edition of the BIPM SI Brochure was published by the BIPM in 2006, with the corresponding new edition of NIST SP 330 published in March 2008. A new edition of NIST Special Publication 811 (SP 811) that reflects the changes incorporated in the 8th edition of the BIPM SI Brochure was published in March 2008 as well. Electronic copies of SP 330 and SP 811 are available online from the Bibliography.

The U.S. delegate to the CGPM is often the NIST Director and the U.S. member of the CIPM is Dr. Willie E. May, Director of the NIST Chemical Science and Technology Laboratory. The current NIST delegate to the CCU is Dr. Ambler Thompson of the NIST Technology Services. Questions concerning the more fundamental aspects of the SI and subtle aspects of proper SI usage may be directed to him at the following address:

Dr. Ambler Thompson
NIST, Bldg. 222, Rm. B243
100 Bureau Drive, Stop 2600
Gaithersburg, MD 20899-2601
Telephone: 301-975-2333
The Metric System
Questions concerning Federal Government use of the SI, Federal Government policy on the use of the SI by U.S. industry and the public, and other issues related to metrication may be addressed to the Metric Program. E-mail:

Additional information
Further discussion of the Meter Convention, the SI, and international metrology may be found at the BIPM web site.

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