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The International System of Quantities (ISQ) is a set of quantities and the equations that relate them describing physics and nature, as used in modern science, officialized by the International Organization for Standardization (ISO) by year 2009.[1] This system underlies the International System of Units (SI), being more general: it does not specify the unit of measure chosen for each quantity. The name is used by the General Conference on Weights and Measures (CGPM) and standards bodies such as the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) to refer to this system, in particular with reference to a system that is consistent with the SI. The ISO standard describing the ISQ by 2009 is ISO/IEC 80000, which replaces the preceding standards ISO 31 and ISO 1000 published in 1992.

Working jointly, ISO and IEC have formalized use of parts of the ISQ by giving information and definitions concerning quantities, systems of quantities, units, quantity and unit symbols, and coherent unit systems, with particular reference to the ISQ. The ISO/IEC 80000 standard defines physical quantities that are measured with the SI units[2] and also includes many other quantities in modern science and technology.[3]

## Base quantities

A base quantity is a physical quantity in a subset of a given system of quantities that is chosen by convention, where no quantity in the set can be expressed in terms of the others. The ISQ defines seven base quantities. The symbols for them, as for other quantities, are written in italics.[4]

The dimension of a physical quantity does not include magnitude or units. The conventional symbolic representation of the dimension of a base quantity is a single upper-case letter in roman (upright) sans-serif[5] type.

Base quantity Symbol for quantity[6] Symbol for dimension SI base unit[6] SI unit symbol[6]
length ${\displaystyle l}$ ${\displaystyle {\mathsf {L}}}$ metre m
mass