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Generally speaking, commas help organize parts of a sentence. While they often signal the natural pauses in the flow of a sentence, they are not breath marks; casually sprinkling commas over a long or convoluted sentence will not make the sentence more readable. Instead, think of commas as (1) links between ideas in a given sentence, whether those ideas are expressed in clauses, phrases, or single words, or (2) dividers that set apart clauses, phrases, or single words.
A. For example, always use a comma to link a dependent (or subordinate) clause to an independent (or main) clause.
Although she enjoyed chemistry, she struggled to stay awake during her quantum mechanics lectures.
B. You should also use a comma to link two independent clauses when you join those clauses with a co-ordinating conjunction (and, but, or, nor, so, for, yet).
She fell asleep during most of her lectures, but she always managed to do well on the exams.
C. However, if the clauses are short, you may omit the comma.
The fire alarm sounded and students filed out.
D. Use commas to link three or more elements in a series – words, phrases, or clauses – that match in grammatical form.
Place the solid in the beaker, stir thoroughly, and stand well back.
A. Commas also set apart various components of the sentence from the main idea. Use a comma to set off an introductory word or a contextualizing phrase.
During their first year at university, most Queen’s students live in residence.
Fortunately, all residence staff members are trained in conflict resolution.
However, if the phrase is short, you may omit the comma.
In spring the birds return to Canada.
B. In the same way, use commas to set apart words, phrases or clauses that add extra but non-essential information to a sentence. These “by the way” or “sidebar” phrases add important details, but omitting them does not change the essential meaning of the sentence. Always frame such phrases with commas.
When she met her assigned roommate, Bob Russell, Shari wondered if Residence Admissions had made a serious mistake.
If your “by the way” information is more complex – a phrase involving internal punctuation, for example – use dashes to set it off from the rest of the sentence.
On your computer, type a dash as two hyphens to distinguish a dash from a hyphen.
- Use a dash to set off an interruption in your sentence.
When she met her assigned roommate – Bob, the very bearded, very friendly, but very clearly male Linguistics major – Shari wondered if the Residence Admissions office had made a serious mistake.
As evidenced in the preceding example, dashes signal a stronger, more emphatic shift than commas do.
- In addition to setting off complex “by the way” phrases, dashes can be used to emphasize a change in thought or emotion in a sentence.
He enjoyed going to the campus pub on Thursday night – but he truly loved attending his 8:30 a.m. biochemistry lecture on Fridays.
- Dashes may also be used to link a list to the main part of a sentence. Although colons also serve this function, you may choose to use a dash when less formality and more emphasis seems appropriate.
After the road trip, the car was crammed with evidence of a successful holiday – burger wrappers, maps, postcards, coffee cups, film canisters, ferry receipts, and one unclaimed sock.