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I'm trying to figure out what style guidelines or rules apply to creating compound adjectives when adverbs are involved. Typically you create compound adjectives when there is potential for ambiguity between the noun and the previous modifiers(s):

This is a high-risk behavior.

But you don't typically hypenate 'ly' adverb pairs (I believe).

This is a highly risky behavior.

While the example above looks correct to me, the example below just feels somehow wrong.

That is a friendly looking dog.

It should be be:

That is a friendly-looking dog.

Is there a general rule or style guidance for when a compound word is preferred over leaving the modifiers separate?

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This post might be useful to you. – Kit Z. Fox Apr 24 '13 at 16:44
"Friendly-looking" is a compound modifier of "dog"; it acts as a unit and deserves to be hyphenated. On the other hand, you would not hyphenate "pretty hungry" in That is one pretty hungry dog. – Jubobs Apr 24 '13 at 16:52
@Jubobs: I doubt that such a sentence is high-frequency enough to merit a discussion about whether to hyphenate "pretty hungry". It's something you'd hear but rarely, if ever, read. And I doubt that any sober native Anglophone would have a lick of trouble understanding the meaning. I also don't think that any compound word "deserves to be hyphenated". Perhaps some "need to be hyphenated" to prevent confusion, but that has zilch to do with merit. – user21497 Apr 24 '13 at 17:13
@BillFranke Less frequent than That is a friendly-looking dog? I'm glad you live in an area where dogs are generally friendly and well fed. Besides, in my experience, hyphenating compound modifiers greatly improves clarity, especially in scientific writing. For that reason, I stand my ground: they deserve to be hyphenated! – Jubobs Apr 24 '13 at 17:18
@BillFranke This discussion about deserve is peripheral, at best, to Robert's question, and SE comments are not an appropriate platform for such a discussion; I will not pursue it here. – Jubobs Apr 24 '13 at 23:32

The general rule in every style manual I know of says don't hyphenate compounds if the first word ends in /-ly/. They also say not to hyphenate foreign phrases like in vitro (e.g., "in-vitro experiments" and "ad-hoc regulations" are both incorrect) because they're set phrases.

A "friendly-looking dog" contains a compound adjective, the first word of which is an adjective ending in /-ly/, not an adverb. "Good-looking", however, is a properly hyphenated compound adjective.

The rules about using a hyphen depend on whether the word is normally hyphenated, in which case it'll be shown as a hyphenated term (or not: not all dictionaries agree) in a good dictionary, and on whether the compound without the hyphen will confuse the reader.

A "friendly looking dog" would not confuse readers by causing them to believe that the dog is a "looking dog" the way some dogs are "hunting dogs": for native Anglophones, in "friendly looking dog", "friendly looking" is clearly a compound adjective, and in "friendly hunting dog", "hunting dog" is clearly a compound noun.

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The problem with hyphenating a compound modifier is when it's composed of elements containing hyphens themselves. Then, all the words in the resulting compound modifier seem to be on the same level, which makes for difficult parsing. I almost wish we used some kind of matching braces as punctuation marks to specify the scope of the modifiers, as in maths or programming. – Jubobs Apr 24 '13 at 17:05
Friendly looking dogs! [Yes, just kidding] – Andrew Leach Apr 24 '13 at 18:04
Brightly-lit as in this is a brightly-lit room would usually be hyphenated. I'd class lit here as a participial adjective; I'd also argue that brightly is a secondary modifier rather than an adverb here (if we're going to analyse the compound at all) - we'd all agree it's not an adjective. I think that it's usual to hyphenate compound adjectives consisting of secondary modifiers ending in -ly (and others), and past- (but rarely present-) participial adjectives. When they're used pre-nominally. eg: well-known, densely-populated, (fast-talking); but a slowly sinking boat (compound?) – Edwin Ashworth Apr 24 '13 at 18:53
@AndrewL: Even "seeing eye dogs" (now replaced by "guide dogs") isn't hyphenated. I wonder whether William Safire would've classified them all as "lookists": they do discriminate based on what things look like. But that's what they're trained to do. Like the rest of us instinctively "lookist" beings. – user21497 Apr 24 '13 at 23:16
@EdwinA: I'm more pragmatic about hyphens: if there's no chance of confusion (assuming common sense rules the reader's mind rather than the pettifoggery of, say, assuming that the magic of your sighs in Will You Still Love Me Tomorrow might just mean "the magic of your size"), don't bother unless the dictionary says it's standard spelling, & use one if there's a chance anyone with an average IQ will be led down the garden path of ambiguity, as in "They listened to more unbelievable stories" ("more unbelievable stories" or "more unbelievable stories"?). – user21497 Apr 24 '13 at 23:28

Diagramming helps. Referring to the last comment: you would diagram seeing-eye on a single line under dog. You would not diagram eye as an adjective below dog with seeing connected to it. As to the original, high-risk is one word describing behavior. In the second sentence, it is risky behavior, and highly tells how risky. The dog would have friendly-looking on one line below it. I hope this helps.

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I've never found diagramming that useful, personally. However, your answer could provide great insight if you provided a actual sentence diagram for one or more example sentences (as opposed to a description) and a link to a reference on sentence diagrams. – Patrick M Apr 8 '14 at 18:31

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