Apostrophes after acronyms and abbreviations

It is common to see an apostrophe used in the plural of an abbreviation or an acronym. For example:

• She had two CJJ's. 
• Buy three CD's for the price of two. 
• Our garage can do twenty MOT's in day. 

As the apostrophes in the examples above do not make the text clearer, we are happy to mark them as incorrect. However, despite what some of the more dogmatic grammarians might tell you, it is not always incorrect to show the plural of an abbreviation or an acronym with an apostrophe.

Apostrophes in Unwieldy-looking Plural Abbreviations and Acronyms

If the writer thinks an apostrophe helps readers, then an apostrophe can be used in a plural. For example:

• She sent two SOS's. 
• I have five SUS's in my care. 

Be aware that using an apostrophe to show the plural of an abbreviation or an acronym is not a popular convention. Lots of your readers will hate it. Many would prefer the versions below. (You are advised to set a high threshold for using an apostrophe to show a plural; i.e. you should try hard to avoid it.)

• She sent two SOSs. 
• I have five SUSs in my care. 

The two examples above are very acceptable, but they do look a little unwieldy. Using an apostrophe to avoid an unwieldy-looking plural is permissible.

Apostrophes in Uppercase Plural Abbreviations and Acronyms

When only capital letters can be used, it is permissible to use an apostrophe to show the plural of an abbreviation or an acronym. (This happens mostly in document titles or advertisements.)

• CDS REQUIRES THREE CD'S 
(The Chief of Defence Staff requires three compact discs.)
• VISITING SUS'S AFTER 1 APRIL 
(Visiting Soldiers under Sentence after 1 April)