Dashes as another form of internal punctuation convey a particular meaning or emphasize and clarify a certain section of material within a sentence. Compared with parentheses, dashes may convey a less formal or more emphatic “aside.”
There are 4 types of dashes that differ in length: the em dash, the most common; the en dash; the 2-em dash; and the 3-em dash. When preparing a manuscript, if symbols for various dashes are not available in the word-processing program, use 2 hyphens to indicate an em dash (--) and 1 for an en dash (-).
Em dashes are used to indicate a marked or pronounced interruption or break in thought. It is best to use this mode sparingly; do not use an em dash when another punctuation mark will suffice, for instance, the comma or the colon, or to imply namely, that is, or in other words, when an explanation follows.
All of these factors—age, severity of symptoms, psychic preparation, and choice of anesthetic agent—determine the patient’s reaction.
An em dash may be used to separate a referent from a pronoun that is the subject of an ending clause.
Osler, Billings, Apgar—these were the physicians she tried to emulate.
The en dash is longer than a hyphen but half the length of the em dash. The en dash shows relational distinction in a hyphenated or compound modifier, 1 element of which consists of 2 words or a hyphenated word, or when the word being modified is a compound.
post–World War I
multiple sclerosis–like symptoms
decision tree–based analysis
phosphotungstic acid–hematoxylin stain
non–small cell carcinoma
The 2-em dash is used to indicate missing letters in a word.
The study was conducted at N—— Hospital, noted for its low autopsy rate.
The 3-em dash is used to show missing words.
Each subject was asked to fill in the blank in the following statement: “I usually sleep ——— hours per day.”
I admire Dr ——— too much to expose him in this anecdote.