A compound adjective comprises two or more words. The unit formed then serves as an adjective. These are also called phrasal adjectives. The vast majority of writers have no clue about spelling these structures. As a general rule, you should hyphenate phrasal adjectives, especially those coming before the word modified. Consider these examples:
a common-law doctrine a civil-rights case a management-consulting firm
One type of compound adjective you never hyphenate is the one formed with an ‑ly adverb and (1) a true adjective, (2) a past participle, or (3) a present participle. Here are three examples:
a newly free nation a widely used procedure the rapidly increasing revenue
If, however, the adverb is not an ‑ly adverb, you do hyphenate the compound adjective if it precedes the modified noun. If it follows the modified noun, do not hyphenate it. Thus:
the well-known actor the actor is well known
For a complete discussion on the use of the hyphen in compound adjectives, see the Punctuation section on Grammar.com. Click here for the beginning of that discussion.