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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?

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.

I recommend the following two articles:

Hyphenated Compound Words

Hyphenated Compounds: When and Why?

Unfortunately, the news in those articles isn't good. The distinction between hyphenated and non-hyphenated compounds is largely driven by convention, and while there are some rules of thumb, you have to be prepared for numerous exceptions and arbitrary differences between countries.

I am not only a published writer, but I am also a trained court reporter and freelance proofreader/editor. Professionally, at least within the career of court reporting, reasons for hyphenating or not hyphenating certain words can vary. What follows are two reasons off the top of my head why one might see certain words (and even the same word) hyphenated one place and not another:

1. 
   

   Deposition agencies can have varied, and seemingly random, protocol for how they would like certain things done — including terminal punctuation of words within brackets or parentheses, citation of legal resources or statutes and, of course, certain words they do not want to see hyphenated. It would be fair to assume that publishing companies and news reporting organizations have similar guidelines that editorial staff are told to follow as well. I believe that different industries and fields of interest have some words commonly hyphenated which may differ from the way it is done elsewhere.

   
   
   

   Note: the rules of punctuation in court reporting can be very different from those of standard business English, creative writing, etc. I fought myself tooth and nail (tooth-and-nail?) trying to un-learn and re-learn rules.

   
   



2. Laziness of writers (depending on what it is that you are reading), who seem to have little care if the reader understands what they are trying to say or not.
   It follows that then, I believe, "readability" is an extremely subjective term; maybe too subjective for some, which might cause issue.

I found the following pages very helpful:

• Compound Words


• Rules for Hyphens Between Words & Hyphens with Prefixes

Both your examples that do not have hyphens are words with negative prefixes.
Words that include a negative (or positive) prefix will usually be written without a hyphen.

Examples: antimatter, indecisive, unwilling, probiotic, and nonaggression.

If the word includes an adjective that is neither directly negative or positive but is instead otherwise descriptive, then you should include a hyphen.

Examples: strong-armed, evil-minded, ill-adviced, and well-intended.

"well" is a borderline case, but it is not 100% positive. It's between neutral and positive, not one of those words you would use as a prefix in the same sense that you use pro, un, in or anti.

I hope this makes sense.

EDIT: Upon reading the above answer I now realize there are further prefixes that are neither positive or negative but still should not include a hyphen. I don't think it would be productive for me to quote the full list but words like extraordinary, infrastructure and transatlantic certainly seem to prove my theory a bit flawed. The point remains, however, that there are certain prefixes that are usually used without hyphen.

There are certainly differences between US and Australian English, where we (Australia) have a tendency to hyphenate more than the US does. This is particularly the case where the un-hyphenated version is difficult on the eye: reenter, reelect, deescalate, and many of similar form.

In my part of the world these would normally all be hyphenated, except perhaps in the Murdoch newspapers, but even that is probably debateable.

I would think we also hyphenate almost all non- words (non-aggression, non-negotiable, etc). I also think hyphenation increases as the second word length increases.

There is an historical element to hyphenation. Compound words we commonly spell unhyphenated now were once two words. Then, as they became more commonly used, they became compound, hyphenated words. Finally, when used enough, they became one word. Usage seems to determine when a hyphen is used. The guide websites cited above suggest "consult a dictionary", but my 1890 Webster's Unabridged and my 1820 copy of Encyclopedia Britannica show very different usages of the hyphen from my current OED or American Heritage. (Some words reverse this process, as the "EnglishPlus" citation suggests, when they fall out of favor: "solid-state" becomes (currently preferred) "solid state".O