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Add an s, but not an apostrophe, to form the plural of most abbreviations:
Use an apostrophe and s to form the plural of numerical names of aircraft ending in a single letter:
In cases where the resulting form would be ambiguous, add an apostrophe before the s:
Add an apostrophe and s to form the plural of abbreviations containing more than one period, and an s without an apostrophe, to form the plural of abbreviations with only one period. In the latter case, the s precedes the period:
The plurals of Mr. and Mrs. are irregular:
The plural forms of the abbreviations for certain bibliographic references are different:
Note that SI/metric symbols maintain the same form for both singular and plural and are written without periods, except at the end of a sentence:
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