The Sentence Lab: which punctuation is legal here?
A full stop, a semicolon and a colon are each decided by one question: can the words beside the mark stand alone as a sentence? Say whether each side can, then try every mark and read why it is legal there or not.
Two sentences, one slot
Sentence 1 of 5
Cover each side in turn. If both can stand alone, a comma on its own will not hold them.
The tide gauge was installed in 1921 the slot its readings were not published for thirty years.
1. Cover one side, then the other. Can each stand alone as a sentence?
Left of the slot
“The tide gauge was installed in 1921”
Right of the slot
“its readings were not published for thirty years.”
2. Try the marks. Which are legal here?
Answer for both sides first.
- Asks whether each side of a slot can stand alone before any mark is offered
- Writes every mark you try into the sentence and says why it is legal or not
- Covers joins, colons, insertions, words that move, and the places that take no mark
- Offers four ready-made choices where a test question would
The whole table
Every verdict in the lab comes from this table. Decide which row a slot is in by covering each side, and the legal marks follow.
| What stands on each side | Legal | The usual wrong answer |
|---|---|---|
| A complete sentence, then another | a comma with a joining word, a semicolon, a full stop | A comma alone (the comma splice), or nothing at all |
| A complete sentence, then one that explains it | a comma with a joining word, a semicolon, a colon, a dash, a full stop | A comma alone. An explanation is still a second sentence |
| A complete sentence, then the list or phrase it promised | a colon, a dash | A semicolon, which needs a sentence after it |
| An opening phrase or clause, then the sentence it opens | a comma | A semicolon or a full stop, which leaves the opener alone |
| A complete sentence, then a phrase that renames or describes it | a comma, a colon, a dash | A full stop, which leaves the phrase alone |
| The end of an insertion that a comma opened | a comma | No mark, or a dash: the two ends of a fence match |
| A subject, then its verb | no mark | A comma where you would pause for breath |
| A name that says which one is meant | no mark | Commas round the name, as if it could be lifted out |
| Two items of a list whose items contain commas | a semicolon | A comma, which no longer shows where an item ends |
Two more rows have no mark to choose. A word such as however or therefore joins nothing: between two sentences the strong mark goes in front of it and a comma after, and in the middle of one sentence it takes a comma on each side. And a comma goes in front of a joining word, never behind it.
How the table was checked
Against the 60 punctuation questions in our own practice tests
In 56 of the 60, the table calls exactly one of the four choices legal, and it is the choice the answer key marks. In 3 it calls the key legal and a second choice as well: those questions turn on meaning or clarity, which a table of punctuation rules does not judge. In 1 the choices change the words, so the table does not apply. An automated test repeats the count, and none of those questions is used in the lab.
The lab’s 36 sentences, by a person
Not yet. Every sentence in every set was worked through in a browser by Claude, an AI assistant, on 14 October 2026. The lab was published on 14 October 2026 before a person had read it. If a sentence here is wrong, tell us.
The sentences were drafted with AI and are original: none is a College Board sentence, and none comes from the question bank behind our practice tests.
Common questions
- Is the Sentence Lab a grammar checker?
- No. It does not read sentences you type. Each sentence in the lab was written for it, and a person recorded what stands on each side of its slot. The lab applies a fixed table of punctuation rules to that. A checker has to guess at the structure of a sentence, and the guessing is the part that goes wrong.
- What is a comma splice?
- Two complete sentences joined by a comma and nothing else. A comma is not strong enough to hold two sentences: use a full stop, a semicolon, or a comma together with a joining word such as and, but or so.
- When is a semicolon correct?
- When a complete sentence stands on both sides of it. Cover the left side, then the right. If either one cannot stand alone, the semicolon is wrong. The other use is between the items of a list whose items already contain commas.
- Does a colon need a complete sentence after it?
- No, before it. What follows a colon can be a list, a phrase or a whole sentence. What comes before it has to be able to stand alone, which is why the words are: a, b and c is wrong and it has three parts: a, b and c is right.
- Is it free?
- Yes, with no account. Nothing you do in the lab is saved or sent anywhere.
The other tools: the full set. For Math: the graphing calculator.
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