My last MM (mechanics moment) focused on colons. Now we’re moving on to semicolons, perhaps the most often misused mark of punctuation in English.

Colons and semicolons look similar, so people tend to get them confused. That’s unfortunate, as they work in very different ways: whereas colons separate general and more specific segments of a sentence, semicolons separate two independent clauses within a single sentence.

An “independent clause” is a statement that could stand alone as a complete a sentence. How can you tell you have one? Easy: you have a subject and a verb. “I walked” is an independent clause, even though it’s short.

So, why would you want to put two independent clauses in one sentence? In general, you’d do it to show that the two clauses are closely related in terms of logic. Maybe they’re cause and effect. Maybe they’re fact and reinforcement. Maybe they’re point and counterpoint. Here are some examples:

1. Joe lay in bed, thinking about how bad his day was going to be. It was Monday, so all the new reports would be landing on his desk; moreover, the boss had scheduled a staff meeting.
2. Joe’s boss was prone to logorrhea; however, that day he was strangely silent.
3. Maybe he had laryngitis; maybe someone’d finally cut out his tongue.

Notice that none of the above situations have that general-to-specific structure we saw with colons.

Often a semicolon is followed by what my ninth-grade teacher called a “flow word” — “however,” “for instance,” “moreover,” “for example,” etc. Flow words help your reader immediately understand how the two halves of the sentence relate to one another. That’s a kindness, but it’s not required, and in some kinds of writing it would probably sound stilted.

What is required is that you have an independent clause on both sides of the semicolon. This rule is non-negotiable. Semicolons allow you to combine two complete sentences into one. They replace a period, not a comma. This mistake is the one I see over and over again: a semicolon with a fragment on one side or the other. I think some people think of the semicolon as a “stronger comma.” It’s not. It’s a “softer period.”

You wouldn’t use a semicolon to introduce a quote or a list (you’d use a colon):

Joe lay in bed, thinking about how bad his day was going to be: a dozen new reports, an interminable staff meeting, sixty-three paper jams, and Sally’s overpowering perfume.

You also can’t use multiple semicolons in one sentence, except in the case of complex lists. See Semicolons, Part 2, for a discussion of that very different use of the semicolon.