hyphenation - When should com­pound words be writ­ten as one word, with hy­phens, or with spaces?

Some compound words are written without hyphens (nonaggression, nonbeliever), some with hyphens (well-intentioned), and others with spaces (post office).

Is there a rule or good guide as to which option should be used?

Answer

In English, there are three types of compound words:

1. the closed form, in which the words are melded together, such as firefly, secondhand, softball, childlike, crosstown, redhead, keyboard, makeup, notebook;




2. the hyphenated form, such as daughter-in-law, master-at-arms, over-the-counter, six-pack, six-year-old, mass-produced;




3. and the open form, such as post office, real estate, middle class, full moon, half sister, attorney general.

For the most part, compound words that are created by adding a prefix are not hyphenated. For example, there are the words anteroom, extraordinary and coordinate. Some exceptions to this rule are (from the link above):

1. compounds in which the second element is capitalized or a number:
   anti-Semitic, pre-1998, post-Freudian


2. compounds which need hyphens to avoid confusion:
   un-ionized (as distinguished from unionized), co-op


3. compounds in which a vowel would be repeated (especially to avoid confusion):
   co-op, semi-independent, anti-intellectual (but reestablish, reedit)


4. compounds consisting of more than one word: (poster's note: these are phrasal adjectives)
   non-English-speaking, pre-Civil War


5. compounds that would be difficult to read without a hyphen:
   
   pro-life, pro-choice, co-edited

Your original example of "well-intentioned" is also explained here:

The other time we must use hyphenation is to join a word to a past participle to create a single adjective preceding the noun it modifies: "a well-intentioned plan," for example, or "a horseshoe-shaped bar."

So, why isn't nonaggression hyphenated? It can be broken into non + aggression, so it is formed by adding a basic prefix onto the noun. In doing so, it breaks none of the exceptions to the rule: "aggression" is not capitalized, hyphenating the term doesn't avoid confusion, a vowel isn't repeated, the compound only consists of 2 words, and it is perfectly readable without a hyphen.