“Do not use semicolons,” Kurt Vonnegut urged, more than once. “They are transvestite hermaphrodites representing absolutely nothing. All they do is show you’ve been to college.”

Ben Dolnick said, “I’ve come to love the awkward things, and to depend on them for easing me through a complex thought.”

Mary Norris, the New Yorker copy editor who just released Between You and Me: Confessions of a Comma Queen, writes: “There is no mark of punctuation so upper-crust as the semicolon.”

Many writers are fond of semicolons; we use them a lot; even when we shouldn’t; and we often don’t know how to use them. (One clue: not the way we just did.)

The semicolon has three main purposes, one of them mechanical and two of them more for clarity and sense. Think of the semicolon as a long comma, or a wimpy period.

The mechanical purpose is to set off items in a series that already contains commas. A simple series has items separated by commas: “He had eggs, bacon, toast and coffee for breakfast.” (We here make a slight nod to the Great Oxford Comma Wars and simply move on.)

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But what if he had “eggs the way he liked them, over easy,” or “bacon, locally raised, of course,” or “coffee, which he always stirred exactly 10 times to blend in the milk”? Now our “simple” series isn’t so much: “He had eggs the way he liked them, over easy, bacon, locally raised, of course, toast and coffee, which he always stirred exactly 10 times to blend in the milk for breakfast.” The commas only confuse.

Bring on the semicolon, which acts as a super comma to separate the related but separate items: “He had eggs the way he liked them, over easy; bacon, locally raised, of course; toast; and coffee, which he always stirred exactly 10 times to blend in the milk for breakfast.”

Associated Press style has called for that final semicolon before “and” since at least the 1970s, though many people seem to think it’s not required.

A second use for the semicolon is to connect two ideas more closely than they would be connected in separate sentences. Take this passage, and see what happens simply by changing the punctuation:

She promised she would make good on the bet. A month later, she did. She promised she would make good on the bet, a month later, she did. She promised she would make good on the bet; a month later, she did.

In the first example, we have two sentences. Each is a perfectly fine sentence, but the period throws up a wall between them. If the writer wants to more closely connect the promise and the fulfillment of the promise, the two sentences can be rendered as one for a different impact. It’s a matter of style and writer’s choice, not grammar.

In the second rendition, though, a comma is not enough to connect them. (That’s called a comma splice.) You could add a conjunction (She promised she would make good on the bet, and a month later, she did.), but that changes the rhythm of the sentence as well as separates the promise and the fulfillment. That defeats the purpose of putting the two sentences together in the first place.

Up rides the semicolon, allowing the sentences to peacefully coexist. It’s like rolling through a stop sign, where you are aware of slowing, but are still moving forward.

It’s the third usage of semicolons that causes more problems. Semicolons can separate long, complex phrases within the same sentence. In this usage, it acts the way it does for the complex series—clearly separating the ideas in those phrases—as well as the way it does by combining two (or more) thoughts into one sentence.

To start with, you want to be sure that the thoughts you’re separating with those semicolons are related. It may be grammatically defensible to write a sentence like, “I went to the gym to lose weight; a guy was really cute,” but why would you want to? The reader has no idea what the connection is between going to the gym and a cute guy.

A good use of a semicolon allows a reader to connect two independent thoughts without losing track of either. The longer the sentence, or the longer each thought, the harder it is for a reader to follow. Think whether you are using the semicolon for effect, or for affect.

Please don’t write something like this, written tongue in cheek, we hope:

There are no upper limits on how many semicolons you can use; in fact, there aren’t set limits on the number of adjectives you can use to describe a noun, or the number of clauses that can be attached to another relative clause, either; grammar doesn’t normally prescribe such limits; leave it to the legal profession to stretch the boundaries of sensibility, though; yes, your example is valid, although it might be prudent to see whether it wouldn’t read better if one of those semicolons were converted to a full stop.

Even when you know the ideas are related, have pity on the poor reader, who can’t take a mental breath until she sees a period. Just break the sentence up with periods; at the very least limit this use of semicolons to once per paragraph.

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Merrill Perlman managed copy desks across the newsroom at the New York Times, where she worked for twenty-five years. Follow her on Twitter at @meperl.