Unfortunately, viewpoints differ slightly.
Forming plurals of lowercase letters
Apostrophes are used to form plurals of letters that appear in lowercase; here the rule appears to be more typographical than grammatical, e.g. “three ps” versus “three p’s.” To form the plural of a lowercase letter, place 's after the letter. There is no need for apostrophes indicating a plural on capitalized letters, numbers, and symbols (though keep in mind that some editors, teachers, and professors still prefer them). Here are some examples:
p’s and q’s = a phrase indicating politeness, possibly from “mind your pleases and thankyous”?
Nita’s mother constantly stressed minding one’s p’s and q’s.
three Macintosh G4s = three of the Macintosh model G4
There are two G4s currently used in the writing classroom.
University of Sussex:
In writing the plurals of numbers, usage varies. Both of the following may be encountered:
If you’re sending mail to the Continent, it’s advisable to use continental 1s and 7s in the address.
If you’re sending mail to the Continent, it’s advisable to use continental 1’s and 7’s in the address.
Here, the first form is admittedly a little hard on the eye, and the apostrophes may make your sentence clearer. In most cases, though, you can avoid the problem entirely simply by writing out the numerals:
If you’re sending mail to the Continent, it’s advisable to use continental ones and sevens in the address.
An apostrophe is indispensable, however, in the rare case in which you need to pluralize a letter of the alphabet or some other unusual form which would become unrecognizable with a plural ending stuck on it:
Mind your p’s and q’s.
How many s’s are there in Mississippi?
It is very bad style to spatter e.g.'s and i.e.'s through your writing.
Without the apostrophes, these would be unreadable. So, when you have to pluralize an orthographically unusual form, use an apostrophe if it seems to be essential for clarity, but don’t use one if the written form is perfectly clear without it. (Note that I have italicized these odd forms; this is a very good practice if you can produce italics.)
University of Delaware:
For Pluralizing Letters, Numbers, Symbols, and Words Used as Terms
- Always remember to dot your i’s and cross your t’s.
- Your G’s look like 9’s.
- He uses too many and’s and uh’s when he speaks.
CCCF Guide to Grammar and Writing:
An apostrophe is also used to form some plurals, especially the plural of letters and digits. Raoul got four A’s last term and his sister got four 6’s in the ice-skating competition. This is particularly useful when the letter being pluralized is in the lower case: “minding one’s p’s and q’s” or “Don’t forget to dot your i’s.” (In a context in which the plural is clear, apostrophes after upper-case letters are not necessary: “He got four As, two Bs, and three Cs.”) It is no longer considered necessary or even correct to create the plural of years or decades or abbreviations with an apostrophe:
- He wrote several novels during the 1930s.
- There are fifteen PhDs on our faculty.
- My sister and I have identical IQs.
(If you wrote Ph.D. with periods, you would add an apostrophe before the pluralizing “s”: Ph.D.'s) If the abbreviation or acronym ends in “S,” it’s a good idea to separate this final “S” from the pluralizing “s” with an apostrophe: SOS’s.
Some even go to the extremes of the Apostrophe Protection Society:
Apostrophes are NEVER ever used to denote plurals!