em dash ダッシュ

The Chicago Manual of Style (15th Edition) defines an em dash as a short typographical rule measuring the width of an em.

The manual further states on page 263 that the em dash, or simply called the dash, is the most commonly used and most versatile of dashes. Some suggested rules that are easy to remember include: One easy rule for English students to learn is that there are no more than two em dashes per sentence: Using more than two em dashes may cause confusion for your reader. For example:

Example 1. Using only two em dashes:

* Prime Minister Koizumi--he had been playing golf--spoke to the reporters with sweat dripping from his face.
* My cat--not really mine, but my wife’s--ran out of the house again.

In the above examples, we can clearly see the sudden change.

Example 2. Using more than two em dashes:

* Prime Minister Koizumi--he had been playing golf--spoke to the reporters--they had been waiting for hours--with sweat dripping from his face.
* My cat--not really mine--it’s actually my wife’s--ran out of the house again.

In these examples, it is confusing and hard to tell where the break in thought occurs. The Chicago Manual of Style has eight more useful references on how to use the em dash in writing. Check them out! (The style guide of the Department of Electrical and Computer Engineering at University of Illinois Urbana-Champaign also has useful information about the em dash. )

3.9.3 Dash There are two types of dashes, the em dash and the en dash. These get their names from the length of the dash. The em dash is the length of the letter m; the en dash is the length of the letter n. Em dash The em dash, which is formed with two hyphens in typewriter fonts as shown below, is used in sentences instead of a comma, a colon, or parentheses to set off information. The dash generally indicates a sudden break in thought that disrupts the sentence structure:

* Dr. Frankenstein--to put it mildly--regretted ever creating the monster.

Dashes can place more emphasis on the enclosed material than would commas or parentheses.

A handbook for technical writers and editors at NASA titled "Grammar, Punctuation, and Capitalization" (ref. NASA SP-7084) has this to say:

Like the comma, the em dash is used both to enclose and to separate, and like the comma, an enclosing dash needs a partner. In fact, enclosing dashes are replacements for enclosing commas in order to add emphasis. Ebbitt and Ebbitt (1982) suggest a useful philosophy toward using the dash:

* If used sparingly, the dash suggests a definite tone, often a note of surprise or an emphasis equivalent to a mild exclamation.
* If used regularly in place of commas, colons, and semicolons, it loses all its distinctiveness and becomes a sloppy substitute for conventional punctuation. At its best the dash is a lively, emphatic mark.

Some examples from this manual include:

3.6.1. Dashes That Enclose

Dashes may replace commas in enclosing interrupting elements and non-restrictive modifiers and appositives. Dashes are appropriate when a comma might be misread, for example, as a serial comma:

* Wrong: The lasant gas, argon, and 3He were allowed to mix for 45 minutes.
* Better: The lasant gas--argon--and 3He were allowed to mix for 45 minutes.

Dashes are also appropriate when the enclosed element contains internal commas:

* Of the lasant gases studied--argon, xenon, krypton, and neon--argon offers the most promise.
* The most promising lasant gas--argon, which is the lightest gas studied--produced laser output power of 4 W.

Use dashes when the enclosed element needs emphasis. Enclose by dashes a complete sentence that interrupts another:

* Wrong: The one-sided spectrum, engineers call it simply "spectrum," is the output of most spectral analyzers.
* Correct: The one-sided spectrum--engineers call it simply "spectrum"--is the output of most spectral analyzers.

Commas are insufficient to enclose an interrupting sentence; dashes or parentheses are required. The choice of commas, dashes, or parentheses to enclose a nonrestrictive or interrupting element depends on the relation of the element to the rest of the sentence and on the emphasis it requires ( Effective Revenue Writing 1, IRS 1962):

* Commas (most frequently used) indicate only a slight separation in thought from the rest of the sentence.
* Dashes emphasize the element enclosed and clarify meaning when the element contains internal commas.
* Parentheses indicate that the enclosed element is only loosely connected to the rest of the sentence and therefore tend to de-emphasize it.

3.6.2. Dashes That Separate

A dash separates a group of antecedents from their pronoun that is the subject of the sentence:

* Argon, xenon, krypton, and neon--these are the possible choices of noble gases for use in nuclear pumped lasers.

In a displayed list, a dash may separate the item from an explanatory statement:

* Support systems for the facility supply the following:

1. Air--The 600-psi system can deliver a flow rate of 300 lb/sec for 3 min.
2. Cooling water--The closed-loop system delivers 450 gal/min at 550 psig.
3. Gaseous propellants--Hydrogen, oxygen, and nitrogen are supplied from tanks at 2400 psia.

A dash may separate two clauses when the second amplifies or restates the first:

* The toughness of pseudo-maraging steel degrades at cryogenic temperatures--at -320 deg F, its Charpy impact energy is 6 ft-lb.

The colon (section 3.4.1) or semicolon (3.15.1) may also be used for this purpose. The dash is less formal than the colon and more emphatic than the semicolon. A dash may precede a phrase like that is, namely, and for example when it introduces a summarizing or explanatory phrase or clause at the end of a sentence, but a comma or semicolon may also be used (Rowland 1962).



If an explanatory clause follows the sentence, a semicolon is necessary; if an explanatory phrase follows, a comma is sufficient. If the explanatory phrase or clause receives enough emphasis by being at the end of the sentence, use a comma or semicolon, whichever is appropriate:

Moderate emphasis:

* Other random processes have average properties that vary appreciably with time, for example, the load demand on an electric power generating system.
* Some random processes are reasonably independent of the precise time; that is, measurements made at different times are similar in their average properties.

A dash would further emphasize the explanatory material:

Emphatic:

* Other random processes have average properties that vary appreciably with time--for example, the load demand on an electric power generating system.
* Some random processes are reasonably independent of the precise time-that is, measurements made at different times are similar in their average properties.

3.6.3. Conventional Uses of the Dash

The dash is used by convention as follows:

To separate a title and subtitle

* Large Space Systems Technology --1984
* Energy Efficient Transport Technology --Program Summary and Bibliography

In vague or open-ended dates

* 1974-- 1980 to 19--

3.6.4. Use With Other Marks

A semicolon, colon, question mark, period, or exclamation point --but not a comma-- supercede and replace a dash; that is, a semicolon, colon, or period may be the "partner" of an enclosing dash.

ダッシュ

ダッ シュ（または「ダーシ」）は、コンマ、セミコロン、コロンの代用としてまったく自由に使うことができる。したがって、その用法を明確に規定することは むずかしいが、主として文中で「思考の一時的中断を示したり別の新たな考え（語句）を注釈的に挿入する場合、あるいはそのような挿入語句を特に強調したい 場合に使う」と考えておくとよい。コンマが無声のポーズに相当するとすれば、おおざっぱに言って、文中のダッシュは「あー」とか「えー」などの間投詞に相 当する。たとえば次のような例である。

If you don’t finish the report by tomorrow -- I mean by 8:00 a.m. tomorrow--then you are fired!

She told me for the first time ever -- that she loved me!

The book was of no use for the exam-- although I enjoyed reading it.

上 例 b) は She told me, for the first time ever, that she loved me! のように、コンマを使った挿入句とすることもできる。これはいわば英文の「普通」の形で、その分だけ挿入句の強調度合いがダッシュを使った場合と比較して 弱くなる（She told meの後に無声の短いポーズを入れてfor the first time everと続けた場合と、She told me の後に uh . . . という「思考の一時的中断」を示す意味不明の音を入れた場合を比較すると、その感じがわかる）。

例文 c) は The book was of no use for the exam, although I enjoyed reading it.（「コンマ」の項目7参照）とするのが普通だが、コンマの代わりにダッシュを使うとalthough以下にほぼ独立文と同じような強さを与える効果 がある。

もっとも、実用文でこういう微妙なニュアンスを出したり、修辞的な効果を狙う必要があるとも思えない。そもそも実用文において「思考を一時的に中断」したり、ある文中に「別の新たな考え（語句）を挿入する」などということが適当だとも思えない。

なお、ダーシには長いダーシ (—) と短いダーシ (-) があり、前者を
“M (= em) dash” と呼び、後者を N (= en) dash と呼ぶ。通常、英文で使われるのは後者のほうである。ただし、タイプライターや一般的なパソコンのキーボードではダーシをダブルハイフン (--) で代用する。この場合、読みやすくするために前後に1文字分ずつスペースを加えることが多い（通常のダーシでは前後にスペースを加えないのが普通）。

More Examples of Usage:

That's Norika Fujiwara—the Queen of Commercials.
あれは、藤原紀香?CMの女王よ。

Yeah, and you know what they say—as Tokyo goes, so goes Japan.
うん。それに何て言われているか知ってるだろう?東京の行く道、日本も行く。

References:
1. Department of Electrical and Computer Engineering at University of Illinois Urbana-Champaign Style Guide
2. Handbook for Technical Writers and Editors, "Grammar, Punctuation, and Capitalization" (ref. NASA SP-7084)


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