Editing Metric system (section)

=== Base and derived unit structure ===
{{Main|Base unit (measurement)}}
{{See also|SI derived unit}}
A base quantity is one of a conventionally chosen subset of physical quantities, where no quantity in the subset can be expressed in terms of the others. A base unit is a unit adopted for expressing a base quantity. A derived unit is used for expressing any other quantity, and is a product of powers of base units. For example, in the modern metric system, length has the unit metre and time has the unit second, and speed has the derived unit metre per second.<ref name=":0" />{{Rp|page=15}} Density, or mass per unit volume, has the unit kilogram per cubic metre.<ref name=":0" />{{Rp|page=434}}