I don't hear many people saying "for goodness' sake" these days. "For conscience' sake" and "for appearance' sake" are pretty much nonexistent in my world, too. In fact, the only "for … sake" expression I hear lately includes a word I wouldn't use in this space even if I could.
So you might think, as I did, that these terms are on the outs. Therefore, you might figure, there's no use worrying about whether they're written with an apostrophe, as in "goodness' sake," with an apostrophe plus an S, as in "goodness's sake," or with neither, as in "goodness sake."
But that assumption would be wrong. Between 1950 and 2008, "goodness sake" doubled in popularity in books published in the United States., according to Google Books' Ngram Viewer. So it's still worthwhile to know how it and similar expressions are written.
The problem is, there isn't much agreement among style guides about how to write "for … sake" terms. They can't even agree how to categorize the issue.
To find recommendations in the Chicago Manual of Style, for example, you have to look under "possessives." In the Associated Press Stylebook, the discussion is filed under apostrophes. In usage guides such as Merriam-Webster's Dictionary of English Usage and Fowler's Modern English Usage, you have to flip through alphabetized listings to the letter S, for "sake."
Their advice, once you find it, is equally confusing.
The Chicago Manual of Style considers these "for blank's sake" expressions to be possessive. So according to Chicago, you would put an apostrophe and S after "conscience" in "conscience's sake" and after "appearance" in "appearance's sake."
However, the guide makes an exception for expressions like "for goodness' sake," where the word before sake ends in an S. Here, Chicago thinks, feels, you should add just an apostrophe but no additional S because otherwise you'd have too many Ss run together.
The Associated Press Stylebook's recommendations are based on pronunciation. A word like Pete, which doesn't end with an S sound, would get an apostrophe plus an S: For Pete's sake. Goodness, appearance and conscience all end with an S sound, even though they don't all end with an S. For the folks at AP, an S sound followed by the S sound in "sake" is reason enough to drop the extra S: For goodness' sake. For appearance' sake. For conscience' sake.
Merriam-Webster's Dictionary of English Usage says most "for … sake" expressions should probably include the extra S. But if you think that logic or even consistency govern this matter, again, you're in for a shocker.
"The apostrophe plus S is only avoided when the S is never added in speech, as when the expression is a fixed idiom (as for goodness' sake and for conscience' sake are)."
That gives us a clue to pronunciation, too. Turning conscience and goodness into possessives, this guide implies, doesn't change how you pronounce them. That is, you don't add an "iz" sound. Just two syllables each in goodness' sake and conscience' sake."
"Garner's Modern American Usage" suggests skipping the extra S after the apostrophe. While "Fowler's Modern English Usage" says even the apostrophe is optional.
You might be wondering: Is all this confusion really necessary? Can't the simple rule we all learned in grade school apply? To form a possessive of a singular noun, always add apostrophe plus S: Bob's house, the dog's tail, the president's men. It works, so why mess with it?
I can answer that question with just two words: goodness's sake. If you follow the basic possessives rule, this expression ends up with four Ss in a row. For most editing styles, which are concerned with visual appeal as well as logic, that's just too many.
So if you want an easy formula, here's my suggestion. Consider these terms possessive, which means they all require an apostrophe. But when your S sounds start piling up, leave out the possessive S.