<P> A physical quantity is a physical property of a phenomenon, body, or substance, that can be quantified by measurement . A physical quantity can be expressed as the combination of a magnitude expressed by a number--usually a real number--and a unit: n u (\ textstyle nu) where n (\ textstyle n) is the magnitude and u (\ textstyle u) is the unit . For example, 6973167492749999999 ♠ 1.674 9275 × 10 kg (the mass of the neutron), or 7008299792458000000 ♠ 299 792 458 metres per second (the speed of light). The same physical quantity x (\ textstyle x) can be represented equivalently in many unit systems, i.e. x = n 1 u 1 = n 2 u 2 (\ textstyle x = n_ (1) u_ (1) = n_ (2) u_ (2)). </P> <P> Symbols for quantities should be chosen according to the international recommendations of ISO / IEC 80000, the IUPAP red book and the IUPAC green book . For example, the recommended symbol for the physical quantity mass is m, and the recommended symbol for the quantity charge is Q . </P> <P> Subscripts are used for two reasons, to simply attach a name to the quantity or associate it with another quantity, or represent a specific vector, matrix, or tensor component . </P> <Ul> <Li> Name reference: The quantity has a subscripted or superscripted single letter, a number of letters, or an entire word, to specify what concept or entity they refer to, and tend to be written in upright roman typeface rather than italic while the quantity is in italic . For instance E or E is usually used to denote kinetic energy and E or E is usually used to denote potential energy . </Li> <Li> Quantity reference: The quantity has a subscripted or superscripted single letter, a number of letters, or an entire word, to specify what measurement / s they refer to, and tend to be written in italic rather than upright roman typeface while the quantity is also in italic . For example c or c is heat capacity at constant pressure . </Li> </Ul>

Physical values in the real world have two components magnitude and