The Internet, with its myriad of acronyms, can be a confusing place.   You’ve got your WSDLs and your H2s and your WYSIWYGs.  So it’s no surprise that people have started to add apostrophes where they don’t belong.

Case in point:

"Open Developers Create API's"

"Open Developers Create API's"

"Open Development Creates Open Data Systems Called API's"

"Open Development Creates Open Data Systems Called API's"

However, my guiding principle on whether to add an apostrophe has always been possession.  Does the API own something, or is it simply a plural?

Confusion arises because the rules (according to AP Style) are different between single letters and multiple letters.  To quote the Plurals section of the Associated Press Stylebook [2001 Edition]:

SINGLE LETTERS: Use ’s:

Mind your p’s and q’s.  He learned the three R’s and brought home a report card with four A’s and two B’s.  The Oakland A’s won the pennant.

MULTIPLE LETTERS: Add s:

She knows her ABCs.  I gave him five IOUs. Four VIPs were there.

And from the same stylebook’s Punctuation Guide listing for Apostrophes: “DO NOT USE: For plurals of numerals or multiple-letter combinations.”

So people may have learned the single-letter rule once upon a time, and incorrectly apply it to the multiple-letter situation.  (Or they may just think that “APIs” looks wrong and try to add some clarity.)

Source: youtube.com (HT to @telegramsam)