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How do you decide whether a compound should be written as one word, separate words, or hyphenated words?

Compounds

A compound is a word or word group that consists of two or more parts that work together as a unit to express a specific concept. Compounds can be formed by combining two or more words (as in double–check, cost–effective, farmhouse, graphic equalizers, park bench, around–the–clock, or son of a gun), by combining prefixes or suffixes with words (as in ex–president, shoeless, presorted, or uninterruptedly), or by combining two or more word elements (as in macrophage or photochromism). Compounds are written in one of three ways: solid (as in cottonmouth), hyphenated (screenwriter–director), or open (health care). Because of the variety of standard practice, the choice among the styles for a given compound represents one of the most common and vexing of all style issues writers encounter.

Compounds in the Dictionary

A good dictionary will list many permanent compounds, compounds so commonly used that they have become permanent parts of the language. However, a dictionary generally will not list temporary compounds, those created to meet a writer's need at a particular moment. Most compounds whose meanings are self–evident from the meanings of their component words also will not be listed in the dictionary, even if they are permanent and widely used.

The Compound–styling Conundrum

When compounds begin to be used widely, there may be significant variation in how writers style them, and it can take years to achieve a high degree of consistency in their format. 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). Although the styling that ultimately takes hold for a compound may be determined by nothing more than editorial preference, there is one pattern that often holds true as new compounds become entrenched in English. 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.

Styling Internet– and Computer–Related Terms (e.g., on–line, website, e–mail)

Internet–related compounds are still so new that their preferred styling remains in flux, with the same compound styled different ways in different publications. Over time, they will likely become more consistent, but what should writers do now? The following list provides the Internet–term stylings that are currently most widely used in professionally edited, published writing.

• E–mail (with a capital E when used as a noun)
• e–mail (with a lowercase e when used as a verb)
• online
• Web site
• Web page
• e–book
• e–tail
• webcam
• webcast/webcaster
• webmaster (often cap)
• dot–com

Text for this article was adapted from Merriam-Webster's Manual for Writers and Editors.