# Fundamental unit

Those physical quantities, which are defined as the basis of a multivariable system, called basic variables. According to the definition of the VIM each base size is set so that it can not be expressed by different base sizes. In a note to this, the VIM specifies that numbers ( "number of entities" ) may be considered as the base size in each size system. The choice of base sizes can be prepared by physical- practical or didactic aspects.

Example:

- The International Size System ( ISQ ) has the following seven base quantities:

A sizing system is always coupled with a corresponding unit system. The number of basic variables determines the degree of size system and the dimensionality of the system of units. For example, the International Size system is a multivariable system of the seventh degree and the corresponding International System of Units (SI ) is a seven -dimensional system of units.

The qualitative properties of a base size are expressed by their dimension. The dimension of a base size is the relevant unit system as the basic unit: realized (even basic unit).

Example:

- The basic size length in the International System of Units base unit of meters and centimeters in CGS system of units. Both the base units are in meters and centimeters respectively represent in their respective size system the dimension of length.

In general, a dimension is always realized by a corresponding coherent unit. A base unit always represents a base size. In addition, however, it can also serve as a cohesive unit for derived quantities of the same dimension.

Example:

- The meter is the International System of Units base unit for the base size length. In addition, he also serves as a (coherent ) derived unit for amount of precipitation, expressed as volume per area.

The above example shows two variables, which are generally considered different size types, but have the same coherent unit and dimension.

After the VIM is a derived variable in a multivariable system size, which is defined as a function of the basic variables, and a derived unit of measurement for a variable derived. It arises as a product of powers of the base units, while a base unit can not be expressed as a product of powers of other base units. Other meanings of the term derived unit play any role for the demarcation to the base unit.

## Sources

- Physical size