If we select a measuring unit for each size, we get a system of measuring unit.
Today we have a unique international system where for each size there is a precisely defined unit of measure. This is called International System of Units or abbreviated SI.
The SI accurately prescribes how to designate physical quantities
and their measurement units. In this way, the unity of communication is achieved in domain of natural sciences, technology, industry, etc.
In SI we have seven basic sizes and many more derived. Each size has its own mark, and also the measuring unit of each size has its own abbreviated mark.
Basic physical sizes:
Out of the seven basic sizes that are listed, we will talk about the first four at the very beginning. To measure the sizes the instruments are required to in order to determines the value of these sizes. Mass is the only size of the above which is cannot be measured, but it can be compared. Derived sizes are calculated from the basic sizes.