Dash

The dash is a punctuation mark consisting of a long horizontal line. It is similar in appearance to the hyphen but is longer and sometimes higher from the baseline. The most common versions are the endash, generally longer than the hyphen but shorter than the minus sign; the emdash, longer than either the en dash or the minus sign; and the horizontalbar, whose length varies across typefaces but tends to be between those of the en and em dashes.
Typical uses of dashes are to mark a break in a sentence, to set off an explanatory remark, or to show spans of time or ranges of values.
The em dash is sometimes used as a leading character to identify the source of a quoted text.

History

In the early 17th century, in Okes-printed plays of William Shakespeare, dashes are attested that indicate a thinking pause, interruption, mid-speech realization, or change of subject. The dashes are variously longer or composed of hyphens ; moreover, the dashes are often, but not always, prefixed by a comma, colon, or semicolon.
In 1733, in Jonathan Swift's On Poetry, the terms break and dash are attested for and marks:

Blot out, correct, insert, refine,
Enlarge, diminish, interline;
Be mindful, when Invention fails;
To scratch your Head, and bite your Nails.
Your poem finish'd, next your Care
Is needful, to transcribe it fair.
In modern Wit all printed Trash, is
Set off with num'rous Breaks⸺and Dashes—

Types of dash

Usage varies both within English and within other languages, but the usual conventions for the most common dashes in printed English text are these:

An em dash or a spaced en dash can be used to mark a break in a sentence, and a pair can be used to set off a parenthetical phrase. For example:

An en dash, but not an em dash, indicates spans or differentiation, where it may replace "and", "to", or "through". For example:
An em dash or horizontal bar, but not an en dash, is used to set off the source of a direct quotation. For example:
A horizontal bar or the em dash, but not the en dash, introduces quoted text.
In informal contexts, a hyphen-minus is often used as a substitute for an endash, as is a pair of hyphen-minuses for an emdash, because the hyphen-minus symbol is readily available on most keyboards. The autocorrection facility of word-processing software often corrects these to the typographically correct form of dash.
Figure dash

The figure dash has the same width as a numerical digit. It is used within numbers, such as the phone number 555‒0199, especially in columns so as to maintain alignment. In contrast, the en dash is generally used for a range of values.
The minus sign glyph is generally set a little higher, so as to be level with the horizontal bar of the plus sign. In informal usage, the hyphen-minus , provided as standard on most keyboards, is often used instead of the figure dash.
In TeX, the standard fonts have no figure dash; however, the digits normally all have the same width as the en dash, so an en dash can be a substitution for the figure dash. In XeLaTeX, one can use \char"2012. The Linux Libertine font also has the figure dash glyph.

En dash

The en dash, en rule, or nut dash '' is traditionally half the width of an em dash.
In modern fonts, the length of the en dash is not standardized, and the en dash is often more than half the width of the em dash. The widths of en and em dashes have also been specified as being equal to those of the uppercase letters N and M, respectively,
and at other times to the widths of the lower-case letters.

Usage

The three main uses of the en dash are:

to connect symmetric items, such as the two ends of a range or two competitors or alternatives
to contrast values or illustrate a relationship between two things
to compound attributes, where one of the connected items is itself a compound
Ranges of values

The en dash is commonly used to indicate a closed range of valuesa range with clearly defined and finite upper and lower boundariesroughly signifying what might otherwise be communicated by the word "through" in American English, or "to" in International English. This may include ranges such as those between dates, times, or numbers. Various style guides restrict this range indication style to only parenthetical or tabular matter, requiring "to" or "through" in running text. Preference for hyphen vs. en dash in ranges varies. For example, the APA style uses an en dash in ranges, but the AMA style uses a hyphen:

En dash range style	Hyphen range style	Running text spell-out
June–July 1967	JuneJuly 1967	June and July 1967
1:15–2:15 p.m.	1:152:15 PM	1:15 to 2:15 p.m.
For ages 3–5	For ages 35	For ages 3 through 5
pp. 38–55	pp 3855	pages 38 through 55
President Jimmy Carter	President Jimmy Carter	President Jimmy Carter, in office from 1977 to 1981

Some style guides recommend that, when a number range might be misconstrued as subtraction, the word "to" should be used instead of an en dash. For example, "a voltage of 50 V to 100 V" is preferable to using "a voltage of 50–100 V". Relatedly, in ranges that include negative numbers, "to" is used to avoid ambiguity or awkwardness. It is also considered poor style to use the en dash in place of the words "to" or "and" in phrases that follow the forms from X to Y and between X and Y.

Relationships and connections

The en dash is used to contrast values or illustrate a relationship between two things. Examples of this usage include:

Australia beat American Samoa 31–0.
Radical–Unionist coalition
Boston–Hartford route
New York–London flight. If dash–hyphen use becomes too unwieldy or difficult to understand, the sentence can be rephrased for clarity and readability; for example, "The flight from New York to London was a pleasant experience".
Mother–daughter relationship
The Supreme Court voted 5–4 to uphold the decision.

A distinction is often made between "simple" attributive compounds and other subtypes ; at least one authority considers name pairs, where the paired elements carry equal weight, as in the Taft–Hartley Act to be "simple", while others consider an en dash appropriate in instances such as these to represent the parallel relationship, as in the McCain–Feingold bill or Bose–Einstein statistics. When an act of the U.S. Congress is named using the surnames of the senator and representative who sponsored it, the hyphen-minus is used in the short title; thus, the short title of Public Law 111–203 is "The Dodd-Frank Wall Street Reform and Consumer Protection Act", with a hyphen-minus rather than an en dash between "Dodd" and "Frank". However, there is a difference between something named for a parallel/coordinate relationship between two people for example, Satyendra Nath Bose and Albert Einstein and something named for a single person who had a compound surname, which may be written with a hyphen or a space but not an en dashfor example, the Lennard-Jones potential is named after one person, as are Bence Jones proteins and Hughlings Jackson syndrome. Copyeditors use dictionaries to confirm the eponymity for specific terms, given that no one can know them all offhand.
Preference for an en dash instead of a hyphen in these coordinate/relationship/connection types of terms is a matter of style, not inherent orthographic "correctness"; both are equally "correct", and each is the preferred style in some style guides. For example, the American Heritage Dictionary of the English Language, the AMA Manual of Style, and Dorland's medical reference works use hyphens, not en dashes, in coordinate terms, in eponyms, and so on. In other styles, AP Style or Chicago Style, the en dash is used to describe two closely related entities in a formal manner.

Attributive compounds

In English, the en dash is usually used instead of a hyphen in compound attributives in which one or both elements is itself a compound, especially when the compound element is an open compound, meaning it is not itself hyphenated. This manner of usage may include such examples as:

The hospital–nursing home connection
A nursing home–home care policy
Pre–Civil War era
Pulitzer Prize–winning novel
New York–style pizza
The non–San Francisco part of the world
The post–World War II era
*
Trans–New Guinea languages
The ex–prime minister
a long–focal length camera
water ice–based bedrock

The disambiguating value of the en dash in these patterns was illustrated by Strunk and White in The Elements of Style with the following example: When Chattanooga News and Chattanooga Free Press merged, the joint company was inaptly named Chattanooga News-Free Press, which could be interpreted as meaning that their newspapers were news-free.
An exception to the use of en dashes is usually made when prefixing an already-hyphenated compound; an en dash is generally avoided as a distraction in this case. Examples of this include:

non-English-speaking air traffic controllers
semi-labor-intensive industries
Proto-Indo-European language
The post-MS-DOS era
non-government-owned corporations

An en dash can be retained to avoid ambiguity, but whether any ambiguity is plausible is a judgment call. AMA style retains the en dashes in the following examples:

non–self-governing
non–English-language journals
non–group-specific blood
non–Q-wave myocardial infarction
non–brain-injured subjects