understand what you mean by GCSE examinations ( GCSE = General Certificate of Secondary Education ), or by DDT ( dichlorodiphenyltrichloroethane ), or by IQ ( intelligence quotient ), or by FM radio ( FM = frequency modulation ). Indeed, in some of these cases, the abbreviated form of the name is far more familiar than the full name. Otherwise, however, you should try to avoid the use of abbreviations in your formal writing. The frequent use of unnecessary abbreviations will make your text irritating and hard to read. So, you should write four ounces ( not 4 oz. ), 80 miles per hour ( not 80 mph ), the Church of England ( not the C of E ), the seventeenth century ( not C17 or the 17th cent. ) and the second volume ( not the 2nd vol. ) It is far more important to make your writing easy to read than to save a few seconds in writing it. There is one exception to this policy. In scientific writing, the names of units are always abbreviated and always written without full stops or a plural s . If you are doing scientific writing, then, you should conform by writing 5 kg ( not 5 kilogrammes , and certainly not * 5 kg. or * 5 kgs. ), 800 Hz ( not 800 Hertz ) and 17.3 cm3 ( not 17.3 cubic centimetres ). There are a number of Latin abbreviations which are sometimes used in English texts.