SAT Boundaries (Punctuation)
Punctuate sentence boundaries, supplementary elements and lists correctly: periods, semicolons, colons, commas and dashes.
Also known as: SAT punctuation, comma splice, semicolon and colon rules.
Key ideas
- Independent clause + independent clause: use a period, a semicolon, or a comma with and/but/so.
- A colon follows a complete sentence and introduces an explanation, example or list.
- Nonessential elements are set off by matching punctuation: two commas, two dashes or two parentheses.
- Don't put a comma between a subject and its verb or between a verb and its object.
Joining clauses
An independent clause can stand alone as a sentence. Joining two of them with only a comma is a comma splice. Fix it with a period, a semicolon, or a comma plus a coordinating conjunction (for, and, nor, but, or, yet, so).
✗ The lab closed early, the samples were moved.
✓ The lab closed early; the samples were moved.
Colons, dashes and extra information
A colon comes after a complete clause: The recipe needs three things: flour, eggs and milk. Extra (nonessential) information must be enclosed by a matching pair: The bridge, completed in 1937, spans the bay. Don't mix a dash with a comma for the same element.
Worked examples
Example 1 · Fixing a comma splice
The Atacama Desert is one of the driest places on ______ weather stations there have never recorded rain.
- Step 1.Both sides are complete sentences.
- Step 2.Use a semicolon (or period).
Answer: Earth; some
Example 2 · Matching punctuation
The telescope—launched in 2021______ has photographed galaxies formed soon after the Big Bang.
- Step 1.The element opened with a dash, so it must close with a dash.
Answer: 2021—it
Common mistakes
- Comma splices between two independent clauses.
- A colon after an incomplete clause ("The ingredients are: …" is tested as wrong when it splits a verb from its object).
- Opening a nonessential element with a dash and closing with a comma.
Try it yourself
Original practice questions. Your answer is checked instantly.
Which choice completes the text so that it conforms to the conventions of Standard English?
Which choice completes the text so that it conforms to the conventions of Standard English?