Sometimes, it’s the little things that count. Little things like whether to use “a” or “the,” for example, or whether to include a comma in a particular spot.

We’ve talked about parenthetical commas, which set off phrases like this one; the comma’s use in restrictive and nonrestrictive clauses; the comma in direct addresses; the comma in appositives; and, everyone’s favorite to argue over, the serial comma.

Those are but a few of the many uses of commas. To understand how a tiny comma can change a big sentence, here’s a crawl that appeared on a local television screen:

Special weather statement, in effect.

It was followed by a warning that a cold front was approaching, to be followed by strong winds and dropping temperatures.

But that comma didn’t belong. By putting it there, it was separating “in effect,” saying, in effect, that something was similar to a weather statement, not that a special weather statement had been issued. “In effect” is an idiom for “in essence.”

Instead, the “in effect” was intended to mean “taking place.”

What may have thrown the writer of that TV crawl was the absence of a verb in that sentence. The verb is implied: “A special weather statement is in effect.” It could have said “There is a special weather statement in effect,” but first, we’ve advised against beginning sentences with “there is”; and second, it’s almost redundant to say “there is” and “in effect.”

Let’s move on to one of the smallest words in our language: “a.” It’s an article, a “function word” that acts as a definer of what follows it: “a man” is more precise than “man,” for example.

But look what happens to this sentence when it is included or left out:

She was little above average. She was a little above average.

The first sentence implies scorn: She’s not much better than the average person is. But the second implies a bit of praise: She beats out the average person by a small amount.

We’ve already talked about whether something is “the” or “a,” but this is different. Most people won’t misunderstand if you call Argo “the movie” or “a movie,” but omitting or including an “a” can be the difference between pass and fail in readers’ eyes.

And before you ask, yes, the same applies to “an.” It’s a given that it’s given the same respect.

Merrill Perlman managed copy desks across the newsroom at The New York Times, where she worked for 25 years.