I only wish that the Editor's Oath included the bit about "...do no harm..". (No doubt this is not relevent for languagehat, but for some others...)

Steve is a tortured soul -- copy editor by day, anti-prescriptivist by night.

Wow. Drastic times call for drastic measures.

doesn't your calling require you to credit the story to the Onion, whence it came?

See the first link? Click on it.

I am puzzled. Pardon my ignorance, but what is wrong with "twentieth century carburettor warmer",
or have I misunderstood? Noun phrases like "twentieth century" have always been usable as adjectives, have they not? Now "mean-spirited" needs its hyphen, but I would not like to see one inserted in, say, "mean spirit mouthings". Maybe this is a difference between UK and USA practice?

Steve is a tortured soul -- copy editor by day, anti-prescriptivist by night.

That's why he has to duck into a telephone booth before he puts the Hat on.

what is wrong with "twentieth century carburettor warmer"

It needs a hyphen. Otherwise, it could be read as "the twentieth of the century carburettor warmers." Sure, that's not a likely reading, but copy editors can't be expected to weigh each multiword phrase for likelihood of misreading; that's why style manuals have rules, and the rule here (simplified for this comment box, obviously) is that if you use a noun phrase as a modifier, you hyphenate it. I'm not sure what your "mean spirit mouthings" is intended to mean; if 'spirit mouthings that are mean,' then it shouldn't have a hyphen, but if 'mouthings of a mean spirit,' then it should. See how important the hyphen is in disambiguation?

Oops, gotta go -- there's a misplaced modifier in South County!
*dashes into telephone booth*

Thanks for the explanation. I wonder if there are any languages that have words, tones or gestures playing the disambiguating role of parentheses in mathematics?

I am covered in shame -- no idea why I missed the link
mea culpa

But anybody who's not writing in Portuguese in South County already has a copy editor in New York to intervene in a misplaced modifier emergency, which should read misplaced-modifier emergency. Although if the poor writer misplaced his modifier emergency that would be far worse.

"Carburettor" looks wrong to me too. "Carburetor" wins a googlefight about 3-1.

'Carburettor' is British, 'carburetor' is American. So maybe that's where the 3-1 comes from. :)

Something tells me his job is safe.

I didn't get a green eyeshade after I took the oath. In protest, I shall use no punctuation at the end of this sentence

theslot.com has an item which might be of interest to you and to your readers: What Exactly Is a Copy Editor? (via Coudal Partners)

"Twentieth century" certainly does NOT need a hyphen any more that "high school" in "high school student" needs a hyphen. if the compound is familiar enough, the hyphen actually clouds the issue more than clarifying it.

What always bothers me about this rule is: what do I do with comppounds that have more than two elements? A nice example would be "Twenty-first-century" ... that can't be right, can it?

Sure, I could just look this up myself, but why bother when I can just bug you. What's the answer? ;)

Yup, I'm afraid that's correct (at least according to Chicago).

Like high school, twentieth century is a ubiquitous and familiar compound.

from Chicago's Q&A website:

There are a few compounds that, in addition to being ubiquitous as open compounds, always seem to go together and are completely unambiguous in any position without the hyphen. Aside from “mental health,” another example might be “physical therapy.” A “physical therapy expert” would tend not to be misread as a skilled psychiatrist who tends toward vigorous bodily movement. In sum, any compound modifier that is not traditionally hyphenated and would not be misread may be left unhyphenated in any position.

That theory is rather spoiled by the fact that the latest 15th edition of Chicago says specifically, right there on p. 305 (under century), "twentieth-century literature, twenty-first-century history, fourteenth-century monastery.... (Adjectival compounds hyphenated before but not after a noun...)."

I'm afraid an explicit directive in the actual manual trumps a vague waving of the hands on the website.