Punctuation tip: Ellipsis vs. em dash, Part II

Welcome back!

Last time, I defined the purpose of the ellipsis, that punctuation mark built of three or four periods in a row. Let's pick right up from where we left off by examining a similar but NOT identical mark, the em dash, and how the two differ.

Em dash
The em dash (—) has many uses, but generally it's only confused with the ellipsis in one scenario: when showing an interruption or other abrupt change in thought. If an interruption is the case, this usually results in an em dash at the end of the sentence or fragment.

     Miss Prudence faced the chalkboard. As soon as her eyes were off us, Bobby whispered, "So I was telling Billy, I wish—"
     "You wish what, Mr. Thompkins? You wish you were not failing this class?" Miss Prudence turned and directed The Look at us.

An abrupt diversion in thought is more likely to occur mid-sentence. Any tangent phrase, translation, example, or other nonessential detail can be correctly added or inserted with em dashes. In this way, em dashes are simply an alternative to commas or parentheses.

When I move to Cincinnati—assuming this job transfer happens—I will be sure to look her up!

Some perennials—chili peppers, lantanas, and snapdragons, for example—behave as annuals when grown outside their ideal climates.

Phil might be a boor, but he is the best man for the supervisory job—it's just unfortunate for the rest of us that he knows it.

As with the ellipsis, there are typographical considerations with an em dash.

1) The em dash does not exist on your keyboard. To render it correctly, hit the hyphen key twice, with no space between (--). Most word processing programs will change a double-hyphen to a proper em dash by default. Like the ellipsis, this auto-format option can be deactivated if you so choose. You may have noticed that, with the exception of the post above, I usually don't take the time to look up the code for a real em dash for this blog. The double-hyphen is just much more convenient to type.

2) If the em dash is the mark you need, do NOT render it by typing space-hyphen-space. When translated by a word processor, the result will likely be an en dash, a different mark with different uses than the em dash. (And yes, the difference between en dash and em dash will be the subject of a separate future post.)

Now it's your turn, readers. What mystifies, fascinates, or thrills you about the ellipsis and the em dash? Can you think of any examples in which using one where the other should be would result in serious (or funny) miscommunication? All ellipsis and em dash comments are welcome!