Two weeks ago I described how commas are used to bracket parenthetical phrases. Dashes can be used in the same way. Dashes add more interest and emphasis to the parenthetical phrase.

Many of them--about half--will remain at home.
I longed--so very much--for a drink of lemonade!

The dashes above could easily be replaced by commas. The following examples are better with dashes than with commas--

He sometimes cooked for his friends a simple dinner--a veal pie and rice pudding.
Five nations--namely, England, France, Germany, Italy, and Russia--were represented in that one army. (If the list of nations was at the end of this sentence, a colon would be a better choice.)

If the dashes were replaced by commas in these two latter sentences, there could be some confusion between the parenthetical and major parts of the sentence.

Use whatever punctuation you think appropriate.

1. Amonton formulated his law relating friction and the normal load 300 years ago; a hundred years later, Coulomb interpreted friction in terms of cobblestones in a rough road--the bigger the stones, the higher the friction.

2. In other words, the friction is not only anisotropic--dependent on the angle of the tip's motion with respect to the domain orientation--but is also asymmetric, that is, different for the trace and retrace of the force microscope tip over the same scan line.

3. We used Brewster angle microscopy to obtain essential information about the tilt angle of the alkyl chains and their tilt direction (azimuthal angle); this technique is based on p-polarized light and allows local determination of these parameters.

4. Our interpretation of the results--the alkyl chains are hexagonally packed and slightly tilted with a uniform tilt direction within each petal.

Without punctuation, these sentences are hard to understand!

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Created April 30, 1998, by Nancy Burnham and Fred Hutson.