My original plan was to collate all the common errors I see in my work and post them in categories, but my innate idleness makes that proposition unworkable. Instead, I’m going to put them up as I run across them, and you can use the cool tag function to find them if you’re interested.
Dash away! dash away! dash away all!
This — is an em dash. It got its name from being the width of an “m” back in the days when type characters were little bits of metal that someone actually set in a line. Its function is to separate—with emphasis—two or three elements of a sentence. In regular text there is no space before or after it. When I say emphasis, I mean a lot of it, more than simple commas, like this, or even parentheses (for stuff that could just as well be left out) can provide. In formal writing, a semi-colon, to denote a change in the flow of a sentence or join two sentences together, or a colon, to precede a list, is preferred.
An em dash can break a sentence in half or in thirds. If you are using two em dashes, the third part of the sentence takes up where the first part left off. If you feel you need more than two em dashes in a sentence, or even in the same paragraph, and your name is Emily Dickinson, please carry on and then get back to that house that seemed a swelling of the ground. Everyone else, knock it off.
This – is an en dash. It has the same etymological history as the em dash. In everyday writing, it combines two words into an adjective that modifies something else so you can avoid the dreaded multiply hyphenated adjectival phrase. Some examples are “Nobel prize–winning author” and “North Carolina–Virginia border.”
In technical writing, an en dash is used in a range in tabular form or between parentheses (15–30 ppm). In regular text, use “from 15 to 30 ppm” instead; do not combine these into “from 15–30 ppm” or “between 15–30 ppm”—use one or the other (see what I did there?).
Its other common use is to denote a relationship between two words, often one that could be written as “Thing 1 to Thing 2″ or “Thing 1 versus Thing 2.” Examples are “dose–response relationship,” “carbon–oxygen bond,” and “cost–benefit analysis.” An en dash is also used to combine two names when they modify a concept, such as “Fischer–Tropsch effect.”
Comments? Questions? Completely impossible combinations you double-dog-dare me (those are hyphens, by the way) to set right? Bring ’em on. I own the ACS Style Guide, and I know no fear.