Is an apostrophe used when forming the plural of letters, figures, abbreviations, acronyms, etc.?
Although an apostrophe may precede the s when forming plurals of letters, signs, symbols, figures, etc., the trend is to omit the apostrophe unless it causes confusion to do so: In this case, there are no if’s, and’s, or but’s. The case study covered programs which were carried out in the late 1940s. (or 1940’s) The majority of personnel here at the laboratory have PhDs. (or PhD’s) A number of GFIs were installed to reduce the potential for massive electrical shock. (or GFI’s) Responses followed by Xs are simply tallied and then ignored. (or X’s or x’s but not xs) Most of the grades issued by the professor were A’s and I’s.
