Boundaries 2: commas, colons, dashes and extra information
Standard English Conventions is about 26% of Reading and Writing (11–15 of 54 questions). Punctuation questions give four choices that differ only in the marks between words, so one rule decides each one.
Extra information needs a pair of marks
Nonessential information (extra detail) is set off from the rest of the sentence by two matching marks: two commas, two dashes, or a pair of parentheses. Test it: remove the extra part. If the sentence is still complete and its basic meaning is the same, the part is extra and needs the pair. If you remove it and the meaning changes or becomes unclear, the part is essential and gets no punctuation at all.
| Type | Pattern | Example |
|---|---|---|
| Commas | A, extra, B | The Registan, a famous square, attracts visitors. |
| Dashes | A—extra—B | The Registan—a famous square—attracts visitors. |
| Parentheses | A (extra) B | The Registan (a famous square) attracts visitors. |
| Essential | no marks | The square that I visited was empty. |
The pair must match
The opening and closing marks must be the same kind. A comma at the start needs a comma at the end; a dash at the start needs a dash at the end; a parenthesis opens and closes. Mixing them (a comma first and a dash later) is always wrong. Using only one mark is also wrong, except when the extra part comes at the end of the sentence (then a final period closes it) or at the beginning.
Essential or nonessential?
Names and descriptions give the clearest test. “My sister Nilufar paints portraits” (no commas) tells us that I have more than one sister and Nilufar is the one who paints. “My sister, Nilufar, paints portraits” (commas) tells us that I have only one sister, and her name is extra. Clauses with “who”, “which” and “that” work the same way: “The students who finish early may leave” (only some students) but “The students, who finish early, may leave” (all of them). “Which” normally begins extra information; “that” normally begins essential information.
Items in a series and adjectives
Use commas to separate three or more items: “apricots, grapes, and walnuts”. A comma separates two adjectives only when they are coordinate, that is, when you could put “and” between them or reverse them: a “warm, sunny day” (a sunny, warm day) but a “small wooden bridge” (not “a wooden small bridge”). Never put a comma between the last adjective and the noun.
Colons and dashes
A colon comes after a complete sentence and introduces a list, an explanation or a quotation: “The kit has three tools: a compass, a ruler and a mirror.” It must not follow a verb or a preposition: “The tools are: a compass” is wrong, and so is “The kit includes: a ruler”. A single dash can work like a colon (“She had one goal—to win”). The sentence before a colon or a dash must be able to stand alone.
Commas that do not belong
Do not put a comma between a subject and its verb (“The tallest mountain in the range, is covered in snow” is wrong, however long the subject is). Do not put a comma between a verb and its object, or between “that” and the words after it. Do put a comma after an introductory phrase or clause: “After the rain stopped, the market reopened.”
The Registan, a famous square in ______ thousands of visitors every year. Three large tiled buildings stand on three sides of its open space, and the square is often lit up in the evening.
- The blank follows the words “a famous square in”. Read each choice into the sentence.
- “The Registan, a famous square in Samarkand, attracts thousands of visitors” has extra information after the name.
- The first comma is already there, so a second comma must close the extra part.
- A leaves the extra part open; C and D use the wrong marks.
Everyone who attended the school’s annual science ______ a certificate and a small notebook. The head teacher handed them out at the end of the day.
- The subject is “Everyone who attended the school’s annual science fair”.
- The verb is “received”.
- Never separate a subject from its verb, however long the subject is.
- Only B has no punctuation.
The experiment had a clear ______ plants given more light grew taller than the others. After three weeks, the difference was easy to see with the naked eye.
- “The experiment had a clear result” is a complete sentence.
- The words that follow explain what the result was.
- A colon introduces an explanation after a complete sentence.
- A is a comma splice, B is a run-on and D mixes a semicolon with “and”.
The village’s oldest tree (a walnut planted long before the school was ______ beside the well. Children climb it every summer, and nobody remembers who first gave it its nickname.
- The opening mark is a parenthesis, so the extra information must close with one.
- “The village’s oldest tree (a walnut planted long before the school was built) stands beside the well.”
- A comma, a semicolon or a dash cannot close a parenthesis.
- Using only one comma for extra informationLook back: did an opening comma or dash already appear? Then the closing mark must be the same kind.
- Mixing a comma and a dashA comma opens with a comma and closes with a comma. A dash opens with a dash and closes with a dash.
- A comma between subject and verbCross out the extra information and the adjectives. If a comma still separates the subject from its verb, remove it.
- A colon after a verb or “such as”A colon must follow a complete sentence. “Our favorite subjects are maths and physics” needs no colon.
- Commas around essential namesIf the sentence needs the name to say which person is meant, give it no commas (“my sister Nilufar” when there are several sisters).
- A comma before the noun in a list of adjectivesPut commas between coordinate adjectives, never after the last adjective before the noun.