英文中破折号分为短破折号（-en破折号）和长破折号（—em破折号）。en破折号相当于英文大写字母“N”的宽度，约为连字符长度的2倍；em破折号相当于英文大写字母“M”的宽度，约为en破折号长度的2倍。在同一个句子中，最多只能用两个成对的或一个单独的破折号。

1.-en破折号（-en dash）

（1）与and，to，或versus同义，用于由两个同等重要、并列关系的词组成的术语。如：

acid-base titration；bromine-olefin complex；

structure-activity relationship；metal-metal bonding；

nickel-cadmium battery；producer-user communication；

cost-benefit analysis；temperature-time curve；

vapor-liquid equilibrium；等等。

但：

①用连字符连接不同颜色的组合（如blue-green）。

②用from...to...和between...and...时，不能用en破折号替代“to”和“and”。

（2）与to或through同义，用于表示三个或多个数字、时间、字母等组成的区间。如：

12-20 months；Figuresl-4；5-50 kg；sections lb-1f；parts C-E；compounds A-I；Lyon and co-workers（23-26）；Lyon and co-workers 23a-d

但：如果数字有负号等符号修饰时，需要用to或through，不可用en dash。如：

20 to+120 K；-145 to-30℃；～50 to 60；10 to＞600 mL；＜5 to 15 mg

-4 to-6℃（错误：-4--6℃）

又：使用from或between时，不可用en dash。如：

from 500 to 600 mL；between 7 and 10 days；with temperatures of-5 to 25℃the...（错误：with temperatures of-5-25℃the...）；with a confidence interval of 0.03 to 0.57...（错误：with a confidence interval of 0.03-0.57...）；with weight changes of-3.5 to+4.7 kg

（3）用于两个同等重要的人名所组成的修饰语。如：

Ziegler-Natta-type catalyst；Jalm-Teller effect；

Debye-HiickeI theory；Franck-Condon factor；

Fisher-Johns hypothesis；Flory-Huggins interaction；

Michaelis-Menten kinetics；Beer-Lambert law；

Lineweaver-Burk method；Mark-Houwink plot；

Diels-Alder reaction；Bose-Einstein statistics；

Bednor-Müller theory；Henderson-Hasselbalch equation；等等。

有时复合性修饰语中的一部分同时含有连字符。如：

the Winston-Salem-Raleigh group of scientists（两座城市：Winston-Salem和Raleigh）；

Columbia-Presbyteran-Brigham cases（两所医疗机构：Columbia-Presbyteran和Brigham）；

Woody-Mia-Soon-Yi episode（三个人：Woody，Mia和Soon-Yi）；

又如：

alkyl-heavy-metal complexes；high-spin-low-spin transition；(www.daowen.com)

metal-metal-bonded complexes；Michaelis-Menten-like kinetics；

retro-Diels-Alder reaction；transition-metal-chalcogen complexes；等等。

（4）用于表示不同组分的溶液或化学键。例如：

hexane-benzene solvent；the-CH2-segment；the C-H distances；C6H5CO-O-CH3；等等。

2.em破折号（—em dash）

（1）表示突然停顿或叙述的转折（破折号在句子中所表示的停顿比逗号明显）。如：

The human race has survived，and the planet seems to have replenished itself—there are fish，ocean，forests—but what kind of society exists in 2195？

Osteoporosis—and the diagnosis is hard to make early without expensive equipment—may be the most common disorder of postmenopausal women in the United States.

These 2 participants—1 from the first group，1 from the second—were tested separately.

Then，when downcutting resumed，the soft fill was removed preferentially and buried promontories such as Kings Point were exhumed—an ancient Tertiary landscape faithfully，if incompletely，restored.

（2）代替逗号或圆括号，使句子的意思更清晰。如：

All three experimental parameters，temperature，time，and concentration，were strictly followed.

改为：All three experimental parameters，temperature，time，and concentration—were strictly followed.

There are shore deposits，gravel，sand，and clay，but marine sediments underlie them.

改为：There are shore deposits—gravel，sand，and clay—but marine sediments underlie them.

（3）用一对破折号来代替一对逗号，强调非限制性的短语或从句，以引起读者的注意。如：

Another attribute of the Mowry Shale—a diagnostic one，and an unmistakable clue to the identity of the formation—is the presence of countless well-preserved fish scales found with little effort on nearly every outcrop（用破折号表达了Mowry Shale的特点和重要性，从而使得句子强而有力。）

但：

在能够使用其他标点符号的情况下，避免用em破折号分隔短语或非限制性从句。如：

错误：Knauth—not Stevens—obtained good correlation of results and calculations.

正确：Knauth，not Stevens，obtained good correlation of results and calculations.

错误：The singly charged complexes—which constituted bands 1 and 3—liberated maleate anion upon decomposition.

正确：The singly charged complexes，which constituted bands 1 and 3，liberated maleate anion upon decomposition.

（4）引导出对前文一系列陈述的总结或说明。如：

Whether we locate meaning in the text，in the act of reading，or in some collaboration between reader and text—whatever our predilection，let us not generate from it a stratijacket.

The close association with chemogenic units and rocks indicative of pauses in active volcanism，the simple mineral assemblage，and the rhyolite doming in some localities—all suggest that the stratiform sulfides developed through fumarolic activity.

The Japanese beetle，the starling，the gypsy moth—these pests all came from abroad.

（5）表示直接引语的来源或编辑的声明。如：

What is a doctor:A licensed executioner.—Mazarinade

Publication of this letter does not indicate that it represents a policy of the American Chemical Society.—The Editor