Compound words can be used as nouns, verbs, adjectives, or adverbs and may be difficult to spell. They can be spelled as one word, two words, or hyphenated, depending on how the word functions and where the word appears in relationship to the word it modifies or its placement in the sentence.
When you aren't sure how to spell a compound word, consult a dictionary. If you can't find the word you're looking for, you can try applying some general principles—explained in the following paragraphs—about compound modifiers.
Compound adjectives
A compound adjective usually consists of two or more words that express a single idea and function as a unit by modifying a noun. Many, but not all, compound adjectives are hyphenated when they appear before nouns: common‐sense answer, cross‐country trip, full‐length mirror, half‐baked scheme, eighth‐grade students, all‐day workshop, self‐conscious behavior; but midweek meeting, secondhand truck, midcareer change, extramural event, nonviolent protest, worldwide circulation, halfhearted support.
When a compound adjective follows a noun, the hyphen is usually omitted: The athlete was top ranked. The driveway was horseshoe shaped. The coat was velvet trimmed.
Using a hyphen is especially important if the compound adjective could be misunderstood by the reader. For example, a fast‐moving van could mean a van that is moving fast or a moving van that is going fast. Your intended meaning must be clear to the reader.
Compound adverbs
Most compound adverbs are written as two words ( distributed all over, going full speed). Those adverbial compounds beginning with over or under are spelled as one word ( overeagerly, underhandedly). Adverbial compounds consisting of spelled‐out fractions are hyphenated ( two‐thirds completed).