Commas separetes words,phrases, or clauses in a series.
- We need books, pencils, and chairs.
- We played tennis, took walks, and went swimming.
Commas set off days of the week, dates, addresses, and geographical names.
- He lives in Chicago, Illinois.
- It happended on friday, October 9, 1970.
Commas set off parenthetical expressions, words in direct address, and oppositives.
- He was, to be sure, an excellent diplomat.
- And so, my friends, you can see the result.
- Santini, our butcher, was hurt recently.
Nonrestrictive clauses do not limit or define, they are parenthetic and are set off by commas.
- Dick, who is clever, passes all his exams
- San Francisco, where we met, is a beautiful city
Restrictive clauses identify or define the antecedent noun. They are not parenthetical and are not set off by commas.
- Any boy who is clever passes all his exams.
- The place where we met is a beautiful city.
Use a comma before a conjunction joining two independent clauses.
- In the north there are many wheat fields, but in the South cotton fields predominate.
- We had trouble reaching him, but at last he answered.
If two independent clauses are closely related in meaning, but are not connected by a conjucntion, join them with a semicolon.
- In the North there aremany wheat field,inthe South cotton fields predominate.
- We had trouble reaching him, at last, however,he answered