Comma can be used to separate items in a list, delineate nonessential content from the body of the sentence, or set off an introductory clause. Writers also use commas to join together independent clauses connected by a conjunction such as "and" or "but." In addition, commas are needed to separate multiple adjectives used to modify a single noun.
When attributing quotes, writers must insert a comma to distinguish the direct quote from the rest of the sentence. If the comma follows the quote, it should be placed inside the ending quotation mark.
Commas also follow elements in a street address. For example, in a sentence including a full address including house number, street name, city and state, the street name and city should each be followed by a comma. The same rule applies to sentences containing full dates, which should include a comma after the weekday, date of the month and year.
In sentences containing direct address, the comma follows the name of the person being spoken to. Note that direct address can occur at the beginning, middle or end of a sentence.
When writing numbers larger than 999, commas must be inserted before every third number, counting from right to left. Some writers omit the comma for numbers with four digits.