Whether you are writing a business proposal or a letter, proper comma use can make your writing clearer. A comma separates parts of a sentence, creating a small pause to help the reader manage the text. Commas can prevent confusion, particularly in lists. Grammar experts don’t always agree on comma rules, though. If you are writing for a particular profession, consult the style manual most frequently used in your area of work.
Place a comma after introductory language in a sentence. For example, "When the prices dropped, he ordered more supplies." Similarly, use a comma to set off language after the main clause. For example, "The outlook is good, thankfully."
Use a comma and a conjunction to join two sentences into one. Conjunctions that you can use for this purpose include and, but, for, or, nor, so and yet. For example, "He studied the market, but he didn’t buy anything."
Use commas to separate three or more items in a series. Under traditional rules, place a comma after each item in the series, except the last item. For example, "She purchased paper, pens, and envelopes." Some grammar experts, particularly those who work in journalism, prefer to leave off the comma before the conjunction opting instead for "She purchased paper, pens and envelopes."
Set off nonessential elements in the middle of a sentence with commas. If you can delete a phrase or clause and the sentence still makes sense, then the phrase or clause is probably nonessential. Place commas before and after the nonessential language. For example, "The pay, on the other hand, is substantial." Use caution if the phrase begins with “who,” as the phrase might be essential. For example, "Employees who are tardy will lose their job." Clauses beginning with “that” are usually essential.
Use commas to separate coordinating adjectives. An adjective describes a noun. If it makes sense to join the adjectives with the word “and” or “but,” then they are coordinating. For example, "She wanted a fast, easy recipe." The word “seafood” is a non-coordinating adjective.
Place a comma between the name of a city and the name of a state. If the state is not the last word in the sentence, place a comma after the name of the state, as well. In dates, do not use a comma if showing only the month and year. However, if you are using month, day and year, place a comma before the year. If a title follows a person’s name, place a comma before and after the title. For example, "Mary Smith, Ph.D., is on the committee."
Use a comma to set off a quotation -- He said, “I am working tomorrow.” If the quote is stated first, the comma goes inside the quotation mark -- “I am working tomorrow,” he said. Commas go before and after the attribution if it occurs in the middle of a quote -- “I am,” he said, “working tomorrow.”