# ISO 31-0

**ISO 31-0** is the introductory part of international standard ISO 31 on quantities and units. It provides guidelines for using physical quantities, quantity and unit symbols, and coherent unit systems, especially the SI. It is intended for use in all fields of science and technology and is augmented by more specialized conventions defined in other parts of the ISO 31 standard. ISO 31-0 was withdrawn on 17 November 2009. It is superseded by ISO 80000-1. Other parts of ISO 31 have also been withdrawn and replaced by parts of ISO 80000.

ISO 31 covers physical quantities used for the quantitative description of physical phenomena. The presentation here is a summary of some of the detailed guidelines and examples given in the standard.

Physical quantities can be grouped into mutually comparable categories. For example, length, width, diameter and wavelength are all in the same category, that is they are all *quantities of the same kind*. One particular example of such a quantity can be chosen as a reference quantity, called the *unit*, and then all other quantities in the same category can be expressed in terms of this unit, multiplied by a number called the *numerical value*. For example, if we write

then "*λ*" is the symbol for the physical quantity (wavelength), "m" is the symbol for the unit (metre), and "6.982 × 10^{−7}" is the numerical value of the wavelength in metres.

This is a particularly useful and widely used notation for labelling the axes of graphs or for the headings of table columns, where repeating the unit after each numerical value can be typographically inconvenient.

A comprehensive list of internationally standardized mathematical symbols and notations can be found in ISO 31-11.