Two words brought together as a compound are often connected with a hyphen, explains the Purdue OWL. A scary movie might be called a hair-raiser; a person who overemphasizes unimportant details might be referred to as a hair-splitter.
While authorities do not always agree, the following uses of the hyphen are generally agreed upon, OWL points out:
- To join two or more words serving as a single adjective before a noun:
One-way street, media-coated slide, well-known author.
Important note: when compound modifiers come after a noun, they are not hyphenated. For example, The author was well known.
- With compound numbers:
- To avoid confusion or an awkward combination of letters. Some examples:
One would re-sign a petition. (Using the hyphen avoids having the reader think the word was “resign,” as in resigning from a job.) Using the hyphen in semi-independent avoids having two letter i’s in a row. The re-treated patients were released makes it clear that patients were treated again, not that they withdrew.
Sometimes hyphenating avoids confusion. For example, if you read “The samples were placed in a light controlled chamber,” you could think that the chamber was not heavy. The hyphen makes it clear that light was controlled in the chamber. More examples:
The equation describes a first-order reaction.
The slow-growing trees had very dense wood.
A high-resolution microscope was used for the analysis.
- With the prefixes ex- (meaning former), self-, mid-, all-:
Ex-husband, self-perpetuating, mid-September, all-inclusive.
- To hyphenate prefixes when either a capital letter or number follows:
Post-Napoleonic Europe, pre-incubation treatment.
- When hyphens are involved in expressing ages, two hyphens are required. Many writers forget the second hyphen, without which the sentences below imply an “old” person or thing:
We have a two-year-old child.
Five-week-old mice were used in the experiment.
Ten two-week-old plants were used in each treatment.
DashHyphen offers an important reminder: “Note that a hyphen never has spaces on either side.” Grammarbook.com repeats that warning while also stressing that “hyphens must never be used interchangeably with dashes.”
Hyphens help writers help their readers make those all-important connections!