Style guides and usage books don't agree where to put the apostrophe in "dos and don'ts" ("do's and don't's"? "do's and don'ts"?). Here's Grammar Girl's recommendation.
Here's a small problem we can address today: The spelling of the phrase "do's and don'ts" is inconsistent because that apostrophe in the word "don't" makes it tricky.
Generally, you don't use apostrophes to make words or abbreviations plural. For example, you don't use an apostrophe in "CDs" (the plural of "compact discs" or "certificates of deposit"), you don't use an apostrophe in "1970s" (all the years from 1970 to 1979), and you don't use an apostrophe in the word "bagels" (the plural of "bagel").
But English does have a few exceptions. For example, you can use apostrophes when they help eliminate confusion, which happens most often with single letters. "Mind your p's and q's" is typically spelled with apostrophes after the letter P and Q, and we write that the word "aardvark" has 3 a's, not 3 as.
The phrase "dos and don'ts" is an especially unusual exception. The apostrophe in the contraction "don't" seems to make people want to use an apostrophe to make "do" plural, but then to be consistent, you'd also have to use an apostrophe to make "don't" plural, which becomes downright ugly because then the word "don't's" has two apostrophes.
Style guides and usage books don't agree on how to handle it. The Chicago Manual of Style and others recommend "dos and don'ts."
The Associated Press and others recommend "do's and don'ts."
The book "Eats, Shoots & Leaves" recommends the consistent but ugly form that uses as many apostrophes as possible: "do's and don't's." I don't usually go to "Eats Shoots & Leaves" for style advice, but I included it here just to show the range of advice that's out there. OK, and to be honest, because the recommendation is so weird!
Here's my advice: Unless your editor wishes otherwise, if you write books, spell it "dos and don'ts"; and if you write for newspapers, magazines, or the web, spell it "do's and don'ts." If you're writing for yourself, you can spell it any way you want. Just be consistent.
I prefer 'do's and don'ts.'