英语语法：228 Colons

by Mignon Fogarty

Today's topic is how to use colons².
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One of my favorite grammar books, titled* Punctuate³ It Right, has a wonderful name for the colon¹: the author calls it the mark of expectation or addition (1). That's because the colon signals that what comes next is directly related to the previous sentence.
Use Colons After Complete Sentences
The most important thing to remember about colons is that you only use them after statements that are complete sentences. Never use a colon after a sentence fragment.
For example, it's correct to say, "Grammar Girl has two favorite hobbies: watching clouds and seeing how long she can stand on one foot." That's correct because "Grammar Girl has two favorite hobbies" is a complete sentence all by itself.
Notice how the items after the colon expand on or clarify what came before the colon. I referred to my favorite hobbies before the colon and then specifically named them after the colon. A quick and dirty way to decide whether a colon is acceptable is to test whether you can replace it with the word namely. For example, you could say, "Grammar Girl has two favorite hobbies, namely, watching clouds and seeing how long she can stand on one foot." Most of the time, if you can replace a colon with the word namely, then the colon is the right choice.
Let's go back to the complete-sentence issue: it would be wrong to say, "Grammar Girl's favorite hobbies are: skiing and reading" because "Grammar Girl's favorite hobbies are" is not a complete sentence by itself. (And, really, who would rather ski than stand on one foot?) You can often fix those kind of sentences by adding the words the following after your sentence fragment. For example, it would be fine to say, "Grammar Girl's favorite hobbies are the following: skiing and reading" because you've made the thing before the colon a grammatically complete sentence by adding the words the following.
Colons and Lists
For some reason, people seem to get especially confused about when to use colons when they are formatting⁴ vertical⁵ lists. Whenever I talk about the complete sentence rule, the next thing people ask is "But what about when you're introducing a list, like a bulleted list or numbered list?"
No need to worry. The rules are the same whether you are writing lists or sentences: use a colon when you could use the word namely and after something that could be a complete sentence on its own.
Space After a Colon
People often argue about how many spaces to put after a period (the short answer is that one space is more common these days), and the problem also comes up with colons. When people used typewriters, the style was to put two spaces after a colon; but now that almost everyone uses computers with word processing software, the common style is to put one space after a colon (2).
 
Colons and Capitalization
Finally, people always want to know whether they should capitalize the first word after a colon. The answer is that it's a style choice, and it depends on what is following the colon. Although the most conservative grammarians say you should capitalize the first word after a colon when the colon introduces a complete sentence, there are a lot of grammarians who say it isn't necessary. Since you never capitalize the first word after a colon if it is introducing something that isn't a complete sentence, I find it easier to adopt the less conservative rule for introducing complete sentences, and then all I have to remember is that the first word after a colon is always in lowercase (unless, of course, it is a proper name or something else that's always capitalized). But if you are writing for someone who uses a style guide, you should check to see if they have a preference because it is a style issue.
So those are the rules about colons for today: use a colon when you could use the word namely, only use a colon after a complete sentence, put one space after a colon, and don't capitalize the first word after a colon (and that's a style choice, so check with your employer or teacher and be consistent).
Colons have other uses such as in writing the time or formatting citations⁶, but those are topics for another day.
References

1.  Shaw, H. Punctuate It Right. New York: HarperPaperbacks, 1993, p. 68.
*Yes, titled is the right word here.
 

1 colon   

n.冒号，结肠，直肠

参考例句：

• Here,too,the colon must be followed by a dash.这里也是一样，应当在冒号后加破折号。
• The colon is the locus of a large concentration of bacteria.结肠是大浓度的细菌所在地。

2 colons   

n.冒号( colon的名词复数 )；结肠

参考例句：

• Enter unlisted values here. Use semi-colons to separate multiple values. 在此处输入未列出的值。用分号分隔多个值。 来自互联网
• Colons separate hours, minutes, and seconds, although all need not be specified. 用冒号分隔小时、分钟和秒，但所有这些内容并非都需要指定。 来自互联网

3 punctuate   

vt.加标点于；不时打断

参考例句：

• The pupils have not yet learned to punctuate correctly.小学生尚未学会正确使用标点符号。
• Be sure to punctuate your sentences with the correct marks in the right places.一定要在你文章句子中的正确地方标上正确的标点符号。

4 formatting   

n.格式化v.使格式化( format的现在分词 )；规定…的格式（或形状、大小、比例等）；安排…的格局（或规格）；设计…的版面

参考例句：

• Select the block of text and-click and choose Default Formatting. 选择目标文字块，点击鼠标右键，选择默认格式。 来自互联网
• There are emerging conventions about the content and formatting of documentation strings. 这里介绍文档字符串的概念和格式。 来自互联网

5 vertical   

adj.垂直的，顶点的，纵向的；n.垂直物，垂直的位置

参考例句：

• The northern side of the mountain is almost vertical.这座山的北坡几乎是垂直的。
• Vertical air motions are not measured by this system.垂直气流的运动不用这种系统来测量。

6 citations   

n.引用( citation的名词复数 )；引证；引文；表扬

参考例句：

• The apt citations and poetic gems have adorned his speeches. 贴切的引语和珠玑般的诗句为他的演说词增添文采。 来自《现代汉英综合大词典》
• Some dictionary writers use citations to show what words mean. 有些辞典的编纂者用引文作例证以解释词义。 来自辞典例句