Unit symbols and physical quantity symbols
In this chapter we have mentioned some unit symbols; e.g., m for metre, kg for kilogram, s for second and so on. We shall meet others later on. It is important to note that these symbols are always printed in roman (upright) type. In contrast, the symbols used to represent physical quantities are printed in italic (sloping) type. Thus, the symbol m represents a particular mass in kilograms, while t represents a time in seconds. Now these italic symbols represent not only the numerical value of the quantity but also the unit in which it is measured. In order to avoid confusion in certain cases it is therefore advisable to specify the unit being used. Hence, for example, when referring to a radius r it is wise to add (in m) or (in nm) as the case may be. To sum up, then, we must always remember to write the unit symbol after a numerical value but not after a symbol representing a physical quantity. For example, we might express a particular mass as, say, 5 kg or simply as m, but never as m kg.