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SAT punctuation: colons, dashes, and the matching-pair rule

A colon needs a complete sentence before it, and extra information needs matching punctuation on both sides. Learn the two rules behind many SAT punctuation questions, with visuals and quick practice.

SPSAT Practice Editorial7 min read

The short version

What you'll take away

  • A colon (or a single dash) must follow a complete sentence and introduces an explanation, example, or list.
  • Extra, nonessential information is set off by a matching pair: two commas, two dashes, or two parentheses.
  • Do not put commas around information that identifies which person or thing you mean, such as “biologist Rachel Carson.”
In this article

The colon rule: complete sentence first

A colon says “here is what I mean.” The part before it must be able to stand alone as a complete sentence. The part after it can be a single word, a list, a phrase, or another complete sentence, as long as it explains or specifies what came before.
A single dash can do the same job, with a slightly more informal or dramatic feel. A semicolon cannot: it needs complete sentences on both sides.

See the skill

A complete sentence, then an explanation that is not a sentence

The experiment measured one thinghow quickly the seedlings grew
  • Colon

    : A complete sentence introduces the explanation.

  • Dash

    — A single dash can also introduce an explanation.

  • Semicolon

    ; The second part is not a complete sentence.

  • No punctuation

    (none) The two parts run together without a break.

Use the visual as a reasoning aid: tie each choice to the relationship, grammar, or evidence the passage actually gives.

The matching-pair rule

Extra information that could be removed without breaking the sentence is called nonessential. It is set off by punctuation on both sides, and both marks must match. If the sentence opens the interruption with a dash, it must close it with a dash.

See the skill

Open and close with the same mark

Two commas

, … ,

The fossil, which was found in 2019, is now on display.

Two dashes

— … —

The fossil—which was found in 2019—is now on display.

Two parentheses

( … )

The fossil (found in 2019) is now on display.

Mismatched pair

✗ — … ,

The fossil—which was found in 2019, is now on display.

Use the visual as a reasoning aid: tie each choice to the relationship, grammar, or evidence the passage actually gives.

Essential information gets no commas

If the information tells you which person or thing the sentence means, it is essential, and commas would wrongly treat it as removable. In “the biologist Rachel Carson wrote Silent Spring,” the name identifies which biologist, so there are no commas around it.
Flip it around and the logic changes. In “Rachel Carson, a biologist, wrote Silent Spring,” the name already identifies her, and “a biologist” is extra description, so it takes a pair of commas.
  • Title or description, then name: no commas (“author Toni Morrison”).
  • Name, then description: a matching pair (“Toni Morrison, the author, …”).
  • Never place a single comma between a subject and its verb.

Put it together

When the choices differ only in punctuation, decide what the punctuation needs to do. Is the sentence introducing an explanation, closing an interruption, or joining two complete sentences? Each job has its own marks.

See the skill

Which mark does this spot need?

  1. Is the text before the blank a complete sentence?

    Cover everything after the blank and read what remains.

    Yes, and an explanation follows

    Colon or single dash.

    No

    No colon. Check for an interruption instead.

  2. Does the blank close an interruption?

    Look left for an opening comma, dash, or parenthesis.

    Yes

    Use the same mark that opened it.

    No

    Check whether two complete sentences meet here.

  3. Two complete sentences?

    Use a period, a semicolon, or a comma with and, but, or so. A comma alone is a comma splice.

This flow covers colons, dashes, and pairs. For joining sentences, see the sentence boundaries article.

Your turn

Quick practice

The researchers focused on a single species of frog ___ glass frog, whose skin is nearly transparent. Which choice completes the text so that it conforms to the conventions of Standard English?

Hint: Is “the glass frog, whose skin is nearly transparent” a complete sentence?

Choose one answer

Practice: try these before checking the answers

Decide the job first: introduce, interrupt, or join.
  • 1. Fix: “The recipe requires: flour, sugar, and eggs.”
  • 2. Fix: “The bridge—completed in 1937, connects the two cities.”
  • 3. Commas or not? “The painter Frida Kahlo created many self-portraits.”
  • 4. Choose: “She had one goal (: / ;) finishing the marathon.”

Sources & further reading

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