1 Introduction

The International System of Units was established in 1960 by the 11th General Conference on Weights and Measures
(CGPM - see Preface). Universally abbreviated SI (from the French Le Systéme International d'Unités)
, it is the modern metric system of measurement used throughout the world.
This *Guide* has been prepared by the National Institute of Standards and Technology
(NIST) to assist members of the NIST staff, as well as others who may have need of such assistance,
in the use of the SI in their work, including the reporting of results of measurements.

The Preface gives the principal Federal Government actions taken since 1988 regarding the SI and introduces the international body - the CGPM - that is responsible for the SI.

A check list is provided to help NIST authors review the conformity of their manuscripts with proper SI usage and the basic principles concerning quantities and units.

A detailed Contents, the aim of which is to simplify the use of the *Guide*, follows the check list.

This introductory chapter gives the purpose of the *Guide* and its outline, while Chapter 2 summarizes and clarifies the NIST policy on the use of the SI in NIST publications.

Chapter 3 notes the existence of a number of publications on the SI and gives the two organizational units at NIST to which questions concerning the SI may be directed and from which additional information about the SI may be obtained.

Chapter 4 discusses the fundamental aspects of the SI, including the two current classes of SI units: base, and derived; those derived units that have special names and symbols, including the degree Celsius; and the SI prefixes that are used to form decimal multiples and submultiples of SI units.

Chapter 5 discusses units that are outside the SI and indicates those that may be used with it and those that may not.
It also gives (see Sec. 5.4) precise definitions of the terms "SI units" and "acceptable units" as used in this *Guide*.

Chapter 6 gives the rules and style conventions for printing and using units, especially unit symbols and SI prefix symbols.

Chapter 7 and Chapter 8, which some readers may view as the most important parts of this *Guide*, provide, respectively, the rules and style conventions for expressing the values of quantities, and clarifying comments on some often troublesome quantities and their units.

Chapter 9 gives the rules and style conventions for spelling unit names.

Chapter 10 further elaborates on printing and using symbols and numbers in scientific and technical documents and is intended to assist NIST authors prepare manuscripts that are consistent with accepted typesetting practice.

Appendix A gives the definitions of the SI base units and the radian and steradian, while Appendix B gives conversion factors for converting values of quantities expressed in units that are mainly unacceptable for use with the SI to values expressed mainly in SI units. Appendix B also includes a simplified discussion of rounding numbers and rounding converted numerical values of quantities.

Appendix C discusses in some detail most of the references included in Appendix D - Bibliography, which concludes the *Guide*.