On a blog I shall not name, I saw the following post title:
China's Rare Earth Advantage
The phrase "rare earth" should have been hyphenated. Why? Because we normally hyphenate phrasal adjectives that precede the nouns they modify. But why? Because the hyphen often helps clear up potential ambiguities. Compare:
"China's rare earth advantage": what is an "earth advantage," and why is it rare?
"China's rare-earth advantage": ah—I get it. China's got an advantage when it comes to rare-earth metals.
See the difference?
Another conundrum, though, is how to capitalize such a title once you introduce the hyphen. The proper way would be thus:
China's Rare-earth Advantage
Why not capitalize "earth"? you ask. Because "earth" is now part of a compound, which removes the justification for capitalization. You can capitalize the second element of the compound only if it's already a proper noun: African-American history.
There are exceptions to the phrasal-adjective rule of hyphenation. The basic rule of thumb is: you can get away with not using a hyphen if there's absolutely no chance of any ambiguity.
Next, I really need to write a post on how to use semicolons. So many lost souls.