Strictly speaking, a hyphen is one type of dash. Dashes come in different lengths and the hyphen is the shortest one. Many people use a hyphen for any and all situations in which a dash is required, but this is bad form and results in unprofessional-looking documents.
There are two other dashes, the en dash, which is middle-sized, and the em dash, which is longest. You can insert these characters from the Special Characters tab or with Ctrl+Numpad - for an en dash and Alt+Ctrl+Numpad - for an em dash (Numpad - refers to the hyphen character on the numeric keypad.)
When should you use which? In addition to their most common use when a word is broken at the end of a line, hyphens are used in the following situations:
- To separate non-inclusive numbers, such as a phone number or a Social Security number.
- To separate compound modifiers, such as dog-eat-dog competition and twenty-four hours.
The en dash is used to connect numbers and words with the implied meaning of "to," "up to an including" or "through." For example, "Her college years were 1996–2000" or "The New York–Miami train is running late."
The em dash, sometimes just called the dash, has many uses. It is an unwritten rule that no sentence should include more than two em dashes to avoid confusion. Its most common uses are as follows:
- To set off an amplifying or explanatory element. "He outlined his business plan—a plan he hoped would save the company" and "The employees—those that could be bothered to show up and vote—approved the change unanimously."
- To separate a subject or series of subjects from a pronoun that refers to them. "Respect—that is what Charles was so desperately seeking" and "Jealousy, bitterness, and spite—such were the difficulties that Marcia faced after her promotion."
Returning now to the shortest dash, the hyphen, we see that there are two special types. They both display the same, but control how lines break. Word breaks lines as needed after a normal hyphen. For example, if the text "dog-eat-dog" is at the end of a line, Word might move "dog" or "eat-dog" to the next line. To prevent this, insert a non-breaking hyphen by pressing Shift+Ctrl+- (use the - key to the left of the Backspace key, not the one on the numeric keypad). Words that are separated by a non-breaking hyphen are always kept together. In Show Formatting mode, a non-breaking hyphen is shown touching the previous character.
The final type of hyphen is the optional hyphen, inserted with Ctrl+-. You insert an optional hyphen to tell Word where to break the text to the next line if needed. If the text does not need to be broken, the optional hyphen is not displayed. If a line break is needed, the hyphen is displayed and the break is placed immediately after it. In Show Formatting mode, an optional hyphen is shown as a dash with a small tail on it.