The suggestion is sometimes advanced that, in written and printed discourse, the spelling-out of units can be dispensed with and symbols can be used exclusively. This idea is practical in genuine measurements, that is, when the symbol is preceded by Arabic numerals. Other cases call for the spelled form: * The spelled form is needed anywhere that a word is referred to as a word - for example, in a dictionary. * A word can be used to discuss a concept - for example, in the history of measurement. * A word is needed to explain its corresponding symbol. * A major advantage in most languages is that a word can be inflected to make, for example, a possessive or plural form. * In European alphabets, the spelling of a word provides a cue to pronunciation. * The spelled-out version can be a pedantic redundancy to prevent mistakes - for example, mega versus milli in pharmacology. * Indexes, including online search engines, include the spelled-out version. Interestingly, they treat symbols as alphabetical. * A unit may need to be spelled out when it is introduced as a courtesy so that the reader doesn¹t have to look it up. Some readers even in this newsgroup might need to look up ³Bq,² for instance. * Computer programs that convert text into voice need the spelled-out version, or milliampere becomes mother. * There are other telling circumstances that I can¹t think of right now.
In most engineering and scientific writing, there is no reason to spell out the unit.
