The comma decisions spell-check will never catch
A spell-checker is good at exactly one kind of mistake: the kind with a right answer. Misspell a word and it knows. Drop a verb and, most of the time, it flags the gap.
But there’s a whole category of comma decision that never trips an underline, because nothing about the sentence is technically wrong. The words are all spelled correctly. The grammar checker has nothing to say. And the sentence still doesn’t mean what the writer thought it meant.
The clearest example I know of ran through a Maine courtroom in 2017. A group of dairy delivery drivers sued Oakhurst Dairy over unpaid overtime, and the entire case turned on one sentence in state law listing the kinds of work exempt from overtime pay: “canning, processing, preserving, freezing, drying, marketing, storing, packing for shipment or distribution of” perishable food.
No comma sat between “shipment” and “or distribution,” which left it genuinely unclear whether “packing for shipment or distribution” described one combined activity or two separate ones, with real money riding on which reading a court chose. Judge David J. Barron opened his ruling for the First Circuit with a line that could sit on a plaque in every newsroom: “For want of a comma, we have this case.” The case went on to end in a $5 million setlement for the drivers.
Nothing in that sentence was misspelled. Nothing was ungrammatical, in the narrow sense a checker tests for. Run it through any spelling or grammar tool available today and it would come back clean, because the software isn’t built to ask what a list of activities actually groups together. The comma that would have prevented years of litigation was a judgment call about what the sentence actually meant, and judgment calls are exactly what automated tools can’t make for you. Here are a few more, the ones I find myself explaining to writers most often.
1. The comma that decides whether a clause defines or just adds detail
“The manuscript that the author revised twice sold within a week” says something different from “The manuscript, which the author revised twice, sold within a week.” Drop the comma and the clause narrows down which manuscript you mean, out of several. Add it and the clause becomes a side note about a manuscript already identified some other way. The Chicago Manual of Style’s own editors put the test plainly: a clause is restrictive “if it can’t be removed without obscuring the identity of the noun it modifies.” Everything else gets set off with commas. A spell-checker sees two grammatically valid sentences and moves on. Only a reader who knows which manuscript you mean can tell you which one you actually wrote.
2. The comma before “because” that decides what’s actually being denied
“I didn’t call because I was busy” and “I didn’t call, because I was busy” look like the same sentence with a stray mark added. They aren’t. The first one, read strictly, denies that busyness was the reason for the silence, implying you didn’t call, and busyness had nothing to do with it, or there’s another reason coming. The second states plainly that you didn’t call, and offers the reason why. It’s a small distinction, and most readers won’t stop to name it consciously, but they’ll feel it anyway, in the small flicker of confusion before the sentence resolves. Writers who deal in apologies, excuses, or explanations run into this constantly without realizing it’s a punctuation choice at all. That flicker is the whole cost of getting it wrong, and no checker will ever flag it, because both versions are perfectly grammatical.
3. The comma that turns a name into an aside
Addressing someone directly changes the shape of a sentence, and the comma is the only thing marking that change. “Thanks Graeme” reads like a label. “Thanks, Graeme” reads like someone speaking to a person in the room. The difference sounds trivial until you’re editing dialogue or a direct-address opening line and the missing comma quietly turns a warm sentence into a flat one. Spell-check has no opinion on tone. It only checks whether “Graeme” is a recognized word.
4. The comma that decides whether the last two items in a list are one thing or two
The Oakhurst Dairy case was really a serial comma problem wearing a lawsuit’s clothes, and most serial comma decisions never make it anywhere near a courtroom. They just quietly decide whether the last two items in a list are separate or combined. “I dedicate this book to my parents, the editor and my dog” reads, to a literal eye, as though the editor and the dog are the parents. Add one more comma before “and” and the ambiguity disappears. Whether to use that comma as a habit or only when a sentence needs it is a house-style choice, and reasonable editors land on both sides of it. What isn’t optional is checking, list by list, whether leaving it out could ever be misread. That check is a judgment call a spell-checker was never built to make.
5. The comma splice that’s a choice, not a mistake
Technically, joining two complete sentences with only a comma is an error, one grammar checkers are quick to flag. In practice, plenty of accomplished writers splice sentences together on purpose, because the pause a comma creates is shorter and quicker than the full stop a period demands, and sometimes that’s exactly the rhythm a sentence needs. The skill isn’t avoiding the splice altogether. It’s knowing the difference between a splice that’s doing something for the reader’s ear and one that’s just two thoughts that never got separated. A checker treats every splice the same way. An editor reads the sentence around it before deciding whether to touch it at all.
None of these five decisions will ever earn a red underline, because none of them are errors in the way a checker understands the word. They’re choices about what a sentence is actually claiming, and making them well is most of what editing a manuscript is, once the spelling and the grammar have already been taken care of. A tool can tell you a sentence is correct. It takes a reader, working through the sentence the way an actual reader would, to tell you what it means.