Semicolons, Colons, and Dashes

Punctuation marks: terribly powerful in the right hands. Punctuation marks are silent allies, and you can train yourself to exploit them as such. Punctuation marks do not just indicate sound patterns—they are symbols that clarify grammatical structure and sentence meaning. And, as I demonstrate in the writing of this paragraph, punctuation marks showcase your facility with the language. What follows are some basics about three of the most powerful and most commonly misused punctuation marks.

The Semicolon

The semicolon is often misused in technical writing; in fact, it is often confused with the colon. Grammatically, the semicolon almost always functions as an equal sign; it says that the two parts being joined are relatively equal in their length and have the same grammatical structure. Also, the semicolon helps you to link two things whose interdependancy you wish to establish. The sentence parts on either side of the semicolon tend to "depend on each other" for complete meaning. Use the semicolon when you wish to create or emphasize a generally equal or even interdependent relationship between two things. Note the interdependent relationship of the two sentence parts linked by the semicolon in this example:

The sonde presently used is located in the center of the borehole; this location enables the engineer to reduce microphonics and standoff sensitivity.

Here, we see how the second half of the sentence helps to explain a key detail (the sonde location) of the first half. The semicolon, along with the repetition of the word "location," helps to draw our attention to the explanation.

The semicolon is also handy for linking a series of parallel items that could otherwise be confused with each other. One savvy student used the semicolon in a job description on her resume as follows:

As an engineering assistant, I had a variety of duties: participating in pressure ventilation surveys; drafting, surveying, and data compilation; acting as a company representative during a roof-bolt pull test.

The Colon

The colon: well-loved but, oh, so misunderstood. The colon is not just used to introduce a list; it is far more flexible. The colon can be used after the first word of a sentence or just before the final word of a sentence. The colon can also be used to introduce a grammatically independent sentence. Thus, I call it the most powerful of punctuation marks.

The colon is like a sign on the highway, announcing that something important is coming. It acts as an arrow pointing forward, telling you to read on for important information. A common analogy used to explain the colon is that it acts like a flare in the road, signaling that something meaningful lies ahead.

Use the colon when you wish to provide pithy emphasis.

To address this problem, we must turn to one of the biologist’s most fundamental tools: the Petri dish.

Use the colon to introduce material that explains, amplifies, or summaries what has preceded it.

The Petri dish: one of the biologist’s most fundamental tools.

In low carbon steels, banding tends to affect two properties in particular: tensile ductility and yield strength.

The colon is also commonly used to present a list or series, which comes in handy when there is a lot of similar material to join:

A compost facility may not be located as follows: within 300 feet of an exceptional-value wetland; within 100 feet of a perennial stream; within 50 feet of a property line.

The Dash

The dash—which is typically typed as two hyphens or as one long bar (available on your word processor’s "symbol" map)—functions almost as a colon does in that it adds to the preceding material, but with extra emphasis. Like a caesura (a timely pause) in music, a dash indicates a strong pause, then gives emphasis to material following the pause. In effect, a dash allows you to redefine what was just written, making it more explicit. You can also use a dash as it is used in the first sentence of this paragraph: to frame an interruptive or parenthetical-type comment that you do not want to de-emphasize.

Jill Emery confirms that Muslim populations have typically been ruled by non-Muslims—specifically Americans, Russians, Israelis, and the French.

The dissolution took 20 minutes—much longer than anticipated—but measurements were begun as soon as the process was completed.

Finally, the dash we typically use is technically called the "em dash," and it is significantly longer than the hyphen. There is also an "en dash"—whose length is between that of the hyphen and the em dash, and its best usage is to indicate inclusive dates and numbers:

July 6–September 17           pp. 48–56.

Like the em dash, the en dash is typically available on your word processor’s symbol map, or it may even be inserted automatically by your word processor when you type inclusive numbers or dates with a hyphen between them. When you type the hyphen, en dash, and em dash, no spaces should appear on either side of the punctuation mark.