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?
Best Answer
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.