Before a conjunction in a compound
sentence:
The boy hit the ball,
but the first baseman caught it.
After items in a series:
I like planes,
trains, and
automobiles
To set off nonrestrictive or
parenthetical elements in a sentence:
George Bush,
president of the United States,
said today he was
making the chad the official symbol of his
administration.
After dependent clauses and certain
words and phrases at the beginning of sentences:
Although I was sick,
I came to class.
In fact, I
was running one hundred degrees of fever.