The SI System of Fundamental Units
Measurement and quantity come in two parts:
The number and
Without both parts the measurement makes no sense. For example the length of a ruler might be 1 metre, but without the 'metre' the '1' makes no sense: it could be 1 foot or 1 yard for example.
In order for a measurement to be understood, the units need to be defined. There are many possible systems of measurement. The one most commonly used is the 'International System of Units' or 'System Internationale', abbreviated to 'SI system'.
In the SI system, all units are written in terms of seven fundamental or base units. These are shown in the table below.
Amount of susbtance
All the SI units are defined in terms of fundamental physical properties of matter. Being properties of matter, they are constant. The mass of an object is measured without reference to a definition independent of place, time, or a standard object to which necessarily only one person can have access at any one time.