http://blog.sina.com.cn/s/blog_63724f620100gdv4.html

1、em dash连接的句子前后都不能有空格。
2、除非遇到天生大写的字母，em dash后接的单词不能大写，例：
Juan tried begging, bribing, and even demanding cooperation
from his staff—all of whom were swamped with other work—before
he gave up and wrote the report himself.
3、en dash和em dash的得名，en dash的长度等于一个typesetter's
letter　N，而em dash的长度则与一个M字母的长度相当。因此，前者
称为en dash，后者称为em dash，都是英文写作中经常用到的破折号。
4、en dash也要求前后不加空格。
5、表示时间、日期、空间等范围时，一律用en dash，而不是hyphen。
hyphen只用于连接单词构成复合词。

例如：Sitting-room, left-handed
It takes 3–6 days to get there.

5、-、–、—　（这三个符号依次为hyphen、en dash、em dash，长
度由左向右递增。可是，看上去第一个与第二个一样长，这是网页
字体格式显示有误所致，copy到写字板或者记事薄中查看就会看到
真实效果了。

6、在记事本或者别的字处理软件中打出这三个符号的方法：

　hyphen（-)：非常简单，就是键盘上＋号前那个符号
　em dash（–）：如果使用笔记本电脑，首先打开Numlock，如果使用
桌面电脑，打开数字锁定键，然后按下ALT，同时按下数字建0150

Guide to authors, a guide for the preparation of Geological Survey of Canada maps and reports
http://ftp.maps.canada.ca/pub/nrcan_rncan/publications/ess_sst/209/209973/of_3600.pdf

Hyphens, En Dashes, Em Dashes

First of all, there are three lengths of what are all more or less dashes: hyphen (-), en dash (–), and em dash (—). I frame it this way because the work they do is roughly related to their length (though I don’t think CMOS puts it this way outright).

The hyphen connects two things that are intimately related, usually words that function together as a single concept or work together as a joint modifier (e.g., tie-in, toll-free call, two-thirds).

The en dash connects things that are related to each other by distance, as in the May–September issue of a magazine; it’s not a May-September issue, because June, July, and August are also ostensibly included in this range. And in fact en dashes specify any kind of range, which is why they properly appear in indexes when a range of pages is cited (e.g., 147–48). En dashes are also used to connect a prefix to a proper open compound: for example, pre–World War II. In that example, “pre” is connected to the open compound “World War II” and therefore has to do a little extra work (to bridge the space between the two words it modifies—space that cannot be besmirched by hyphens because “World War II” is a proper noun). Now, that is a rather fussy use of the en dash that many people ignore, preferring the hyphen.

The em dash has several uses. It allows, in a manner similar to parentheses, an additional thought to be added within a sentence by sort of breaking away from that sentence—as I’ve done here. Its use or misuse for this purpose is a matter of taste, and subject to the effect on the writer’s or reader’s “ear.” Em dashes also substitute for something missing. For example, in a bibliographic list, rather than repeating the same author over and over again, three consecutive em dashes (also known as a 3-em dash) stand in for the author’s name.

http://www.chicagomanualofstyle.org/qanda/data/faq/topics/HyphensEnDashesEmDashes.html

Hyphen

For most writers, the hyphen’s primary function is the formation of certain compound terms. The hyphen is also used for word division, which is briefly explained here. Never use a hyphen in place of an en dash or an em dash.

http://www.thepunctuationguide.com/hyphen.html

符号大全:破折号怎么打
在中文输入法中，输入ctrl+ -(减号键)可以打出破折号。

http://www.fhdq.net/bd/298.html  

The grammar school version (for English usage) is:

• - (known as an hyphen) between the elements of compound words

• -- (known as an en-dash) for ranges "3--7"

• --- (known as a em-dash) punctuation for digressions in a sentence---though how it differs from a parenthetical comment I have never known---which is why you don't see it much

  http://tex.stackexchange.com/questions/3819/dashes-vs-vs

No. 9 连字符Hyphen，短破折号En dash，长破折号Em dash

a. 连字符Hyphen: Used as a "connector"

例子：

1. 把前缀或者后缀和词干连接起来： ex-husband, self-inflicted, meta-analysis, etc.

2. 连接复合词： light-year, cure-all

3. 连接第二个元素是过去或现在分词的复合修饰语：

seizure-inducing drugs, well-known physicist, well-established rules

4. 当不使用连字符时可能不明确时连接复合修饰语：

low-frequency amplitudes, a large-bowel obstruction

5. 连接修饰语与数值和单位。年龄术语采用双连字符：

a 5-g dose, 50-km radius, a 3-year-old child

6. 连接拼写的分数：

one-third of the population, thirty-two hundredths

7. 当拼写出来时，将从21到99的复合基数词和复合序数词连接起来：

Eighty-five samples were collected.

He pointed to the Sixty-Sixth Congress.

8. 连接需要连字符的才能得到正确含义的动词：

a) He re-covered the explored well.

...BUT He recovered quickly from the operation.

b) Such patients are usually re-treated. ...BUT The water retreated from the structures.

9. 在引用中连接页码范围： 454-455, 35-37.

10. 连接所谓的“电子术语”，其中“e”代表“电子”： e-mail, e-commerce,

e-business

b. 短破折号En dash： 大写字母"N."的长度，主要用于连接，在大多数情况下意味着“through”。

例子：

1. link 2 words representing items of equal rank, including compound modifiers. In this case, the en dash means "and" or "to":

north–south avenues, cost–benefit analysis, hexane–benzene solvent, author–editor

relationship

2. 以齐名的方式连接2个人的姓名：

Mann–Whitney U test,

Michaelis–Menten kinetics

3. 连接包含连字符元素的词语：

sugarmaple–dominated forest

c. 长破折号Em dash： 大写字母"M."的长度，用作分隔符，用于在句子中强烈的中断。

例子：

1. 像逗号和括号一样使用：Cancer—the major cause of death in this population—is often difficult to detect in the early stages.

http://blog.sciencenet.cn/blog-288924-1048271.html