Ends a declarative sentence, and — in American usage — sits inside closing quotation marks.
Spacing
No space before the period; exactly one space after it before the next sentence begins.
Two spaces after a period is a typewriter-era habit, not a rule. Every current style guide (Chicago, AP, MLA, APA 7th) specifies one.
In abbreviations that end a sentence, the abbreviating period doubles as the sentence period — you never type two.
Placement
Inside closing quotation marks in American usage, regardless of whether the period belongs to the quoted material.
Outside closing quotation marks in British usage when the period is not part of the quotation.
Inside a closing parenthesis only when the whole sentence sits inside the parentheses; otherwise outside.
Right and wrong, side by side
Correct
The meeting ended at four. Everyone left quickly.
Incorrect
The meeting ended at four. Everyone left quickly.
One space after the sentence-ending period, not two.
Correct
She arrived at 9 a.m.
Incorrect
She arrived at 9 a.m..
The period in the abbreviation also closes the sentence. Never double it.
Correct
He called it "a complete disaster."
Incorrect
He called it "a complete disaster".
American convention puts the period inside the closing quotation mark even when it is not part of the quoted words.
Correct
The proposal was rejected. (Nobody was surprised.)
Incorrect
The proposal was rejected. (Nobody was surprised).
A complete sentence inside parentheses keeps its own period inside the closing parenthesis.
Worth knowing
The one-space rule is the single most common thing people get told off for and the single most common thing people are wrong about being told off for. Typewriters used monospaced type, where a wider sentence gap genuinely helped the eye; proportional digital type does that spacing itself. If you are writing for a publisher with its own house style, follow the house style — but the default is one.
Runs in your browser. It checks what is mechanically decidable from the characters — spacing, doubling, pairing, dash-for-hyphen — and two clause-level patterns. It does not judge whether a comma is well placed, because that is not decidable from the characters.
Pick a punctuation mark and this tool lays out what it actually does: how it spaces, where it sits relative to quotation marks and brackets, and what separates a correct use from the near-miss that looks almost identical. Each mark comes with right and wrong versions of the same sentence placed next to each other, because the difference between "He called it 'a complete disaster.'" and "He called it 'a complete disaster'." is invisible when you read the rule as prose and obvious when you see the two side by side. Fourteen marks are covered, from the period and the comma through the three dashes people routinely collapse into one, to the ellipsis — including the four-dot form, which is not an ellipsis with an extra dot but a sentence's own period followed by a three-dot ellipsis, and which confuses people for exactly that reason. Underneath the reference there is a checker: paste your own sentence and it reports the punctuation problems that can actually be determined from the characters on the page.
How it works
The reference half is straightforward — hand-written records for each mark, covering its spacing rules, its placement rules, its worked examples, and the thing about it that most often goes wrong in practice, with a link through to this site's full guide on the underlying rule. The checker half is where the design decisions are, and the governing one is that it only reports what is mechanically decidable. Punctuation splits neatly into two kinds of question. Some are decidable from the characters alone: whether there is a space before a comma, whether the brackets pair up, whether a run of dots is two or three or five, whether a hyphen has been typed where a dash belongs, whether a semicolon has been followed by a conjunction that makes it redundant, whether a colon has been dropped between a verb and its objects. Those are facts about the text, and the checker reports them without hedging. The other kind is not decidable that way at all. Whether a particular comma helps the reader, whether a dash would be better than a pair of brackets, whether a sentence would be improved by being two sentences — these depend on meaning, rhythm and intent, and a tool that guessed at them would be wrong often enough that you would stop reading its output. So it does not guess at them. Two clause-level checks sit at the boundary and are worth explaining, because both are guarded hard. The first is the comma splice, where a comma joins two sentences that each stand alone. To fire, it requires the sentence to contain exactly one comma — so a parenthetical pair cannot trip it — the sentence not to open with a subordinating word like "although" or "because", which would make the first clause dependent and the comma correct, and the words after the comma to be a subject followed by a finite auxiliary, the one shape that is unambiguously a second main clause. The second is the conjunctive adverb: "however", "therefore" and their relatives are adverbs rather than conjunctions, so they cannot join two independent clauses with commas alone, and "The data was clear, however, nobody acted" needs a semicolon before the adverb. Both of those checks are narrow by construction. They will miss real comma splices written in shapes the guards exclude. That is the deliberate trade: a check that fires on correct writing teaches people to ignore it, and an ignored check is worth less than no check at all. Findings come back marked as errors or as style points, and the distinction is real rather than decorative. An unclosed bracket, a space before a comma, a colon between a verb and its objects — those are errors in any style guide. Two spaces after a full stop, a spaced hyphen standing in for a dash, four dots where three belong — those are style points, defensible in some house styles and simply dated in others, and they are labelled that way rather than dressed up as mistakes.
Why some rules have two correct answers
Several of the marks here behave differently on either side of the Atlantic, and the reference says which convention it is describing rather than pretending there is one rule. Commas and periods go inside closing quotation marks in American usage regardless of whether they were part of the quoted words; British usage puts them inside only when they belong to the quotation, which is the more logical arrangement and the less common one. The em dash is set closed up with no spaces in Chicago style, while British and most news styles use a spaced en dash for the same job. Neither convention is wrong. What is wrong is using both in one document, and that is the only thing the checker will actually flag about them.
The four-dot ellipsis, since it is the question people arrive with
Four dots is correct, and it is not an ellipsis with an extra dot. It is a sentence's own full stop followed by a three-dot ellipsis, which is why it only appears where the material before the omission is a complete sentence. "The committee reviewed the evidence. … No further action was taken" takes four marks in total because "The committee reviewed the evidence" stands alone. Cut words out of the middle of a sentence and you get three, because the fragment before the cut is not a sentence and has no period of its own to keep. That is the entire test: cover everything from the dots onward, and count four if what remains would take a full stop. Five or more dots is not a form of anything, and the checker treats it as an error rather than a style point.
Frequently asked questions
Is four dots ever correct?
Yes, whenever the words before the omission form a complete sentence. The first dot is that sentence's own period and the remaining three are the ellipsis. If the material before the dots would not take a full stop on its own, use three. Five or more is not standard punctuation in any style guide.
Should there be one space or two after a full stop?
One. Every current style guide — Chicago, AP, MLA, APA — specifies one, and has for decades. The two-space habit comes from monospaced typewriters, where every character occupied the same width and a wider gap genuinely helped the eye find the next sentence. Proportional digital type handles that spacing itself. The checker marks two spaces as a style point rather than an error, because a publisher with its own house style may still ask for it.
What is the difference between a hyphen, an en dash and an em dash?
They are three different characters doing three different jobs. A hyphen joins words into one unit and never takes spaces: a well-known author. An en dash spans a range or links two equal things: pages 45–52, the London–Edinburgh line. An em dash marks an interruption or an aside: the proposal—all ninety pages of it—arrived late. Typing a hyphen for any of the other two is the single most common punctuation substitution in everyday writing.
Will the checker tell me whether my commas are well placed?
No, and that is deliberate. Whether a comma helps a reader depends on meaning and rhythm, which cannot be determined from the characters on the page. The checker reports what is decidable — spacing, pairing, doubling, dash substitutions — plus two clause patterns with tight guards. Everything else is left to you and to the rule pages this tool links out to.
Why did it not flag a comma splice I know is there?
The comma-splice check is guarded so it cannot fire on correct sentences, and those guards cost it real cases. It needs the sentence to have exactly one comma, not to begin with a subordinating word, and to have a subject plus a finite auxiliary after the comma. A splice written in any other shape passes silently. That trade is on purpose: a check that flags correct writing gets ignored, and then it catches nothing at all.
Does the tool send my sentence anywhere?
No. The mark records ship with the page and the checks run in your browser. The mark you selected and the sentence you checked are held in the page address so the result can be linked, and that link goes nowhere unless you send it.