For many quantities, you need units which you get by combining the basic units. These units are called **derived units**. For example, combination of the units of length and time gives the derived unit of speed or velocity, m s^{–1}.

One of the advantages of the SI system of units is that they form a coherent set in the sense that the product or division of the SI units gives a unit which is also the SI unit of some other derived quantity. For example, product of the SI units of force and length gives directly the SI unit of work, namely, newton-metre (Nm) which has been given a special name **joule**.