compound unit symbols. For example, is an SI unit of density, where is to be interpreted as (). Coherent and non-coherent SI units When prefixes are used with the coherent SI units, the resulting units are no longer coherent, because the prefix introduces a numerical factor other than one. The one exception is the kilogram, the only coherent SI unit whose name and symbol, for historical reasons, include a prefix. The complete set of SI units consists of both the coherent set and the multiples and sub-multiples of coherent units formed by using the SI prefixes. For example, the metre, kilometre, centimetre, nanometre, etc. are all SI units of length, though only the metre is a coherent SI unit. A similar statement holds for derived units: for example, , , , /, etc. are all SI units of density, but of these, only is a coherent SI unit. Moreover, the metre is the only coherent SI unit of length. Every physical quantity has exactly one coherent SI unit, although this unit may be expressible in different forms by using some of the special names and symbols. For example, the coherent SI unit of linear momentum may be written as either or as , and both forms are in use (e.g. compare respectively here:205 and here:135). On the other hand, several different quantities may share the same coherent SI unit. For example, the joule per kelvin (symbol ) is the coherent SI unit for two distinct quantities: heat capacity and entropy; another example is the ampere, which is the coherent SI unit for both electric current and magnetomotive force. This is why it is important not to use the unit alone to specify the quantity. Furthermore, the same coherent SI unit may be a base unit in one context, but a coherent derived unit in another. For example, the ampere is a base unit when it is a unit of electric current, but a coherent derived unit when it is a unit of magnetomotive force. As perhaps a more familiar example, consider rainfall, defined as volume of rain (measured in ) that fell per unit area (measured in ). Since / , it follows that the coherent derived SI unit of rainfall is the metre, even though the metre is also the base SI unit of length. Permitted non-SI units There is a special group of units that are called "non-SI