Commas
The comma is the workhorse of English punctuation, and most comma errors aren't really about the comma itself — they're about not knowing which of several distinct comma jobs a sentence is asking for. There's no single 'comma rule'; there are close to a dozen separate uses, each with its own logic, and confusing one for another is what produces most bad commas.
Separating items in a series
Commas separate three or more items in a list: I bought apples, oranges, and bread. The comma before the final 'and' is the Oxford (or serial) comma, covered in its own guide — but the commas between the earlier items aren't optional in any style: 'I bought apples oranges and bread' is simply wrong regardless of house style on the last comma.
Joining two independent clauses with a coordinating conjunction
When a comma joins two complete sentences, it needs a coordinating conjunction (and, but, or, nor, for, so, yet) right after it: She studied all night, but she still felt unprepared. Drop the conjunction and keep the comma, and you've written a comma splice (its own separate error, covered elsewhere) — the comma alone isn't strong enough glue to hold two full sentences together; it needs the conjunction's help.
Introductory elements
A comma follows an introductory word, phrase, or clause before the main clause starts: Frankly, I don't see the point. After the meeting ended, everyone left quickly. Having finished the report, she went home. Short introductory phrases (under about four words) are sometimes left uncommaed in informal writing — 'In 2020 the company moved' is acceptable without a comma — but longer or clause-length introductions almost always need one for the reader to find the seam between setup and main clause.
Nonrestrictive elements: commas around information you could delete
This is the rule most people apply inconsistently without realizing it's a rule at all. If a descriptive phrase or clause could be dropped without changing which specific thing the sentence refers to, it's nonrestrictive and gets commas on both sides: My brother, who lives in Denver, called yesterday (I have one brother; the Denver detail is extra). If the phrase is necessary to identify which thing you mean, it's restrictive and gets no commas: The brother who lives in Denver called yesterday (implying I have more than one brother, and this clause tells you which one). This exact logic governs the that/which distinction covered in its own guide.
Setting off appositives, parenthetical information, and direct address
Appositives (a noun phrase that renames another noun) take commas when nonrestrictive: My favorite author, Toni Morrison, won the Nobel Prize. Parenthetical asides do too: The results, frankly, surprised everyone. And so does direct address — naming the person you're talking to: Thanks, Maria, for the update. All three are really the same nonrestrictive-element rule wearing different clothes: the material between the commas is removable.
Dates, addresses, and titles
In month-day-year dates, a comma separates the day from the year and another follows the year if the sentence continues: On July 4, 1976, the country celebrated its bicentennial (note both commas — a frequent error drops the second one). Month-year alone doesn't need a comma: July 1976 was a hot month. In addresses written in running text, commas separate each element: She lives at 12 Elm Street, Springfield, Ohio, and works downtown (again, note the comma after the state when the sentence continues). Academic and professional titles following a name take commas: Maria Chen, PhD, will present the findings.
Where commas commonly go wrong
The comma splice (joining two full sentences with just a comma, no conjunction) is the most cited error, but a nearly as common mistake is the opposite: leaving out a needed comma before a coordinating conjunction joining two independent clauses, which forces a reader to re-parse the sentence. Another frequent error is over-commaing restrictive clauses — sticking commas around a clause that actually identifies which noun you mean, which subtly changes the sentence's meaning even though it looks like a small stylistic slip. And a comma should never separate a subject from its verb, even when the subject is long: 'The report that the committee spent six months preparing, was rejected' is wrong — no comma belongs before 'was' there, no matter how far the subject has wandered from the verb.
Commas with coordinate adjectives
When two or more adjectives independently modify the same noun and could be reordered or joined with 'and' without changing the meaning, they're coordinate and need a comma between them: a long, exhausting meeting (you could say 'an exhausting, long meeting' or 'a long and exhausting meeting'). When adjectives build cumulatively, each one modifying the phrase that follows rather than the noun directly, no comma belongs between them: a bright red bicycle (you can't say 'a red bright bicycle' or 'a bright and red bicycle' — 'bright' modifies 'red bicycle' as a unit, not 'bicycle' alone). Testing whether 'and' can be inserted, or the adjectives reordered, without sounding off is the quickest diagnostic for which case you're in.
Commas with direct quotations
A comma typically separates a quotation from its attributing tag: She said, 'I'll be there by noon.' 'I'll be there by noon,' she said. When the quotation is a question or exclamation, its own punctuation replaces the comma: 'Are you coming?' she asked (no comma needed before 'she asked' since the question mark already does that separating work). When a quotation is woven grammatically into the surrounding sentence rather than formally introduced, no comma is needed at all: She told him it was 'about time' he showed up.
The Oxford comma question, briefly
Whether the comma before the final 'and' or 'or' in a list (red, white, and blue vs. red, white and blue) is required is a genuinely unsettled house-style question rather than a fixed grammar rule — some major style guides require it, others omit it by default — and it's substantial enough to deserve its own dedicated discussion rather than a quick aside here.
Commas with 'which' clauses and other nonrestrictive markers
Because nonrestrictive clauses in English are conventionally introduced by 'which' rather than 'that' (in most edited American prose), a comma almost always precedes a nonrestrictive 'which' clause: The report, which took six months to write, was well received. Leaving out that comma is one of the more common signals that a writer meant the clause restrictively but used the wrong relative pronoun to say so — see the dedicated that/which guide for the full distinction, since the choice of pronoun and the choice of comma placement are really two sides of one decision.
Commas in numbers and measurements
In American usage, commas separate groups of three digits in large numbers: 1,200; 45,000; 2,500,000. Many other countries use a period or a space for this instead (1.200 or 1 200 in much of continental Europe), which is why international documents sometimes standardize on non-comma thousand separators to avoid ambiguity with the decimal point. This numeric use has nothing to do with sentence-level comma rules; it's a separate typographic convention that happens to reuse the same mark.
When commas change meaning entirely
Because commas mark the boundary of a nonrestrictive element, adding or removing one can genuinely change what a sentence claims, not just its rhythm. 'The students, who studied hard, passed the exam' claims all the students studied hard and all passed. 'The students who studied hard passed the exam' claims only some students studied hard, and it was specifically those students who passed (implying others may not have). Neither version is a punctuation error in isolation — the comma's presence or absence is doing real semantic work, which is exactly why sloppy comma placement around this kind of clause is worth double-checking rather than treating as a purely cosmetic choice.
Frequently asked questions
Do I always need a comma before 'because'?
No. 'Because' clauses are usually restrictive (they explain why, and that reason is central to the sentence's meaning), so no comma: She left early because she felt sick. A comma before 'because' is only used in the rarer case where the clause is a nonrestrictive aside or where omitting the comma would create ambiguity about what's being negated, such as 'He didn't leave because he was angry' (ambiguous) vs. 'He didn't leave, because he was angry' (clarifies he stayed, and anger is the reason he stayed).
Is a comma splice ever acceptable?
In certain literary or stylistic contexts — short parallel clauses, especially in fiction — writers deliberately use comma splices for rhythm (I came, I saw, I conquered). In standard expository or professional prose, though, it's treated as an error, and the safer options are a semicolon, a coordinating conjunction, or a period.
Should there be a comma before 'too' at the end of a sentence?
Modern style guides mostly say no comma is needed: I like that movie too. A comma before a sentence-final 'too' (I like that movie, too) is an older convention that's fallen out of favor in most style guides, though it isn't flatly wrong — it's simply optional now rather than required.