Please–don't do it. Don't use apostrophes to make expressions plural, like this:
This photo of a plumbing supply company storefront is NOT for a company owned by people named Spa and Pool. If it were, Spa's & Pool's might be the perfect sign.
Spas & Pools–that's what the sign should say.
Google knows better. When I typed Spa's and Pool's into the search box, it responded:
Did you mean: spas and pools
Yes, Google, the owner of the company meant Spas & Pools–the sign is simply wrong.
You may be wondering whether it is ever correct to use an apostrophe to form a plural. Yes, occasionally that use makes sense. These rules and examples are taken from The Gregg Reference Manual (Gregg), with additional notes from Garner's Modern American Usage (Garner's) and The Associated Press Stylebook (AP).
- Capital letters and abbreviations ending with capital letters are made plural by adding s alone. Examples: three Rs, two Cs, HMOs, FAQs, Ph.D.s, DVDs
However, use an apostrophe before the s where confusion might otherwise result. Examples: three A's, too many I's, two U's (The plurals might otherwise be seen as As, Is, and Us.)
AP agrees with Gregg.
Garner's recommends an apostrophe with capitalized abbreviations that contain periods. Example: M.B.A.'s.
- For the sake of clarity, uncapitalized letters and most uncapitalized abbreviations are made plural by adding an apostrophe and an s. Examples: dotting the i's, wearing pj's, p's and q's, bbc's
Garner's agrees with Gregg, citing gif's as an example.
AP does not discuss uncapitalized letters; for single letters, it uses the apostrophe, citing p's and q's.
- If the plural form is unfamiliar or is likely to be misread, use an apostrophe and an s to form the plural. Examples: or's or nor's, which's and that's (rather than ors or nors, whiches and thats).
AP does not use an apostrophe in these situations, just simple plurals. Garner's prefers using typography rather than apostrophes in these situations, with the word itself in italic type and the s in roman type. Example: Trim the number of ofs to tighten prose.
Everyone agrees that if the singular form contains an apostrophe, add only an s to form the plural. Examples: ain'ts, don'ts
When I learned the rules of plurals many years ago, I think I learned to add an apostrophe to make numbers plural, things like 1970's and temperatures in the 80's. My copy of the Handbook of Business English, published in 1914 (No, I'm not that old! I got it from my 100-year-old cousin), says to use the apostrophe to indicate the plural of figures.
However, these days we use only an s to make numbers plural: 1970s, 80s, W-2s, 1099s, MP3s, etc. Gregg, Garner's and AP agree on these number plurals without apostrophes. Still, the New Oxford Style Manual points out that "find all the number 7's" may be useful. Without that apostrophe, readers might look for the number 7 with a letter s after it.
In my business writing classes, the word that people most often render incorrectly is memos, into which they have inserted an apostrophe. But memos is the correct plural. The term memo's is correct only when you intend it to be possessive: The memo's subject line is too long.
Except for the few exceptional situations above, don't use an apostrophe to form a plural. It will grab your readers' attention, but not for a good reason.
If you and your team argue about apostrophes and other punctuation marks, why not take my self-study course Punctuation for Professionals together?
Feel free to share your bad example's examples of apostrophes for plurals here.