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A comma is a form of punctuation used to separate distinct elements in a sentence, including listed items, dependent and independent clauses, transition words and phrases, and non-essential information.
Commas in a list
Use commas to separate items in lists of three or more items.
e.g., I'm studying Italian, Computer Science, Statistics, and Functions.
Commas and introductory phrases
Commas are used after short introductory phrases. A comma indicates that the introductory information is over, and the main part of the sentence is beginning. Introductory phrases may include context about times or dates. They may also be transition words or phrases.
e.g., In 1949, Newfoundland joined Confederation.
e.g., However, many citizens remained loyal to the idea of independence.
Grammatical Tip: use two commas for transition words used in the middle of the sentence.
e.g., Most of Smith's conclusions, however, are valid.
Commas and coordinating conjunctions
Commas generally come before coordinating conjunctions that join independent clauses.
e.g., I was failing Calculus, so my parents hired a tutor.
e.g., Students today must be better prepared than ever, for competition in the workplace is fierce.
Grammatical Tip: the coordinating conjunctions can be remembered through the acronym F.A.N.B.O.Y.S.
F - FOR
A - AND
N - NOR
B - BUT
O - OR
Y - YET
S - SO
Commas and parenthetical expressions
A parenthetical expression adds secondary or supplemental information to a sentence. Placing commas around this information indicates that the information is non-essential from a grammatical standpoint and could be removed without interfering with the overall meaning or structural completeness of the sentence.
e.g., My PSYCH 345C textbook, which costs over $175, is difficult to understand.
e.g., Marjorie, Don's wife of thirty years, planned a surprise party for his 75th birthday.
Grammatical Tip: commas used for parenthetical expressions can also be replaced by dashes or, occasionally, parentheses.
Commas and complex sentences
Complex sentences are ones that contain one independent clause (complete sentence) and one or more dependent clauses. Whether or not you use a comma depends on the order in which the clauses are presented.
- Use a comma when a dependent clause is followed by an independent clause.
e.g., Because the course was so popular, the department decided to run extra sections in the fall.
- Do not use a comma when an independent clause is followed by a dependent clause.
e.g., The department decided to run extra sections of the course in the fall because it was so popular.
General Tip: while the guidelines in this handout are designed to help you understand and use commas correctly, it is important to note that there will be exceptions to these rules for stylistic or flow purposes. For example, the following sentence is grammatically correct based on comma rules, but it has so many commas in a short space that the sentence may appear disjointed:
e.g., She should be here at 9, but, if not, we can start anyway.
The above sentence might be written to omit the second comma. Be careful, since some people may judge this as grammatically incorrect. However, it is a common construction when a parenthetical expression (if not) directly follows a coordinating conjunction (but):
e.g., She should be here at 9, but if not, we can start anyway.
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