According to my 1933 Oxford Universal Dictionary, "good-bye" and "co-operate" are hyphenated, neither "leg room" nor "birth rate" can be run together into single word, and "teenager" doesn't exist.

But open a current dictionary like Webster's New World and you'll get a very different story. "Goodbye" is usually one word, "cooperate" is also a closed term, and so are "legroom," "birthrate" and "teenager."

As regular readers of this column know, words change over time. These twists and turns of the English language can be hard to navigate and even harder to predict. But when it comes to open compounds, which are terms made up of multiple words separated by spaces, closed compounds, which run words together into one, and hyphenated compounds, the future is easier to foresee.

"With frequent use, open or hyphenated compounds tend to become closed (on line to on-line to online)," the Chicago Manual of Style says.

In other words, as people get used to putting "cell" in front of "phone," they stop thinking of it as a new twist on the familiar phone and start thinking of it as a distinct thing, a cellphone. For nouns especially, the more common the term, the more likely it will end up as a closed compound.

But the part of speech is key. "Compound nouns are usually written as one word, compound verbs are generally written as two, and compound adjectives are very often written with a hyphen," notes Merriam-Webster's Collegiate Dictionary. That's why in the dictionary you'll find the adjective "good-looking" hyphenated, the verb "pick up" open and the noun "makeup" one word. But there are plenty of exceptions: "Cooperate" is a closed compound even though it's a verb.

Other times, it's up to you.

"For many terms, it is often completely acceptable to choose freely among open, hyphenated and closed alternatives, even though the term has been used in English for an extended period (for instance, lifestyle, life-style, or life style)," notes Merriam-Webster's.

The cautious approach is to check a dictionary. If a term isn't listed as closed or hyphenated, then you write it as open. But remember that dictionaries often disagree. For example, according to Merriam-Webster's, "smartphone" is closed and "cell phone" is open. But Webster's New World disagrees on both points.

Here, based on my own highly subjective experience, are some terms to watch out for.

Log out. This is always open, which is confusing when you consider that the verb "log in" has a hyphenated noun form, "log-in," according to Merriam-Webster's. It's one of many computer terms that confuse people. "Internet–related compounds are still so new that their preferred styling remains in flux, with the same compound styled different ways in different publications," Merriam-Webster's notes.

Health care/healthcare. Merriam-Webster's, which book publishers use, says the noun is open. Webster's New World, which a lot of news media use, says it's closed. For the adjective form, then, you would hyphenate it in book style, "a health-care plan," but use the closed form in news media style, "a healthcare plan."

Day care isn't a closed term yet. Leave the noun form open and consider hyphenating the adjective: a day-care center.

Crowd source. The closed form isn't recognized by most dictionaries yet.

Website/Web site. This term is closed and begins with a lowercase "w" in Webster's New World, but it's open and begins with a capital "W" in Merriam-Webster's.

Guest room. People in the hospitality industry consider this a prominent concept worthy of its own closed word. But the leading dictionaries disagree.

Best-selling. As an adjective, it's always hyphenated. The noun form is closed in news style (bestseller) and open in book publishing (best seller).

Prime time. The noun hasn't closed up yet.

Longtime. This adjective is closed, unlike the adjective "long-term."

Everyday/every day. The adjective is closed, "everyday values." The noun phrase is open, "We offer values every day."

Keep your eye out for these troublemaking terms and, when in doubt, check a dictionary.

JUNE CASAGRANDE is the author of "It Was the Best of Sentences, It Was the Worst of Sentences." She can be reached at JuneTCN@aol.com.