Why do some abbreviations require a period, such as M.D. and others do not, such as CIA?
This might help you as well:
Full stops/Periods in abbreviations: Americans tend to write Mr., Mrs., St., Dr. etc., while British will most often write Mr, Mrs, St, Dr, etc., following the rule that a full stop is used only when the last letter of the abbreviation is not the last letter of the complete word; this kind of abbreviation is known as a contraction in the UK. Many British writers would tend to write other abbreviations without a full stop, such as Prof, etc, eg, and so on (so recommended by some Oxford dictionaries). However, the "American" usage of periods after most abbreviations is also widely used in the UK. In either case, it is incorrect to put a stop / period after unit symbols such as kg or Hz; however, in non-scientific contexts, the unit for "inch" is often written in., as it would be ambiguous without the period.
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