ABSTRACT
This chapter explain the term 'SI base units' and describes the seven SI base units. It also explains the term 'SI derived units' and explores the relationship between SI 'base' and 'derived' units. The chapter also describes recognise SI prefixes and provides apply correct SI symbols. It explains use correct SI prefixes and also provides apply SI conventions when writing SI units. SI specifies how its units of measurement should be written. These rules, or conventions, apply to the units themselves, to their symbols and to their associated numerals. Celsius and litre are certainly 'metric' units and in common use, but they are not SI units. They are classified as units that 'may be used alongside' the SI system. The SI unit for temperature is the kelvin, and the SI unit for volume is the cubic metre. The metric system existed long before the introduction of SI.
