While a period ends a sentence, a comma indicates a smaller break. Some writers think of a comma as a soft pause—a punctuation mark that separates words, clauses, or ideas within a sentence.
When a parenthetical element — an interjection, adverbial modifier, or even an adverbial clause — follows a coordinating conjunction used to connect two independent clauses, we do not put a comma in front of the parenthetical element.
The Yankees didn't do so well in the early going, but frankly, everyone expects them to win the season. [no comma after "but"]
The Tigers spent much of the season at the bottom of the league, and even though they picked up several promising rookies, they expect to be there again next year. [no comma after "and"]
When both a city's name and that city's state or country's name are mentioned together, the state or country's name is treated as a parenthetical element.
We visited Hartford, Connecticut, last summer.
Paris, France, is sometimes called "The City of Lights."
You could think of this as "That tall, distinguished, good looking fellow" rule (as opposed to "the little old lady"). If you can put an and or a but between the adjectives, a comma will probably belong there. For instance, you could say, "He is a tall and distinguished fellow." So you would write, "He is a tall, distinguished man." But you would probably not say, "She is a little and old lady."
Peter Coveney writes, "The purpose and strength of the romantic image of the child had been above all to establish a relation between childhood and adult consciousness."
If an attribution of a quoted element comes in the middle of the quotation, two commas will be required. "The question is," said Alice, "whether you can make words mean so many things."
Be careful not to use commas to set off quoted elements introduced by the word that or quoted elements that are embedded in a larger structure: Peter Coveney writes that "[t]he purpose and strength of . . ."
Typographical Reasons: Between a city and a state [Hartford, Connecticut], a date and the year [June 15, 1997], a name and a title when the title comes after the name [Bob Downey, Professor of English], in long numbers [5,456,783 and $14,682], etc.