A transition connects pieces of writing and shows the relationship between them. Transitions may appear between paragraphs, sentences, or parts of a sentence. They include conjunctive adverbs such as also, however, meanwhile, and therefore and transitional phrases such as for example, in addition, in fact, and on the other hand.
If a transition starts a sentence or appears in the middle of an independent clause, set it off with commas:
A heat wave has rolled over Southern California; the Station fire, meanwhile, is burning in San Gabriel Valley.
As a matter of fact, residents of Altadena, Glendale, and Pasadena have been evacuated.
If a transition connects two independent clauses, use a semicolon before it and a comma after:
The fires are close to Los Angeles; in fact, plumes of smoke can be seen from Hollywood.
There are exceptions, of course. (There always are.) If a transition flows with the rest of the sentence without a pause, no commas are necessary:
Certainly both the heat and air quality are concerns for Southern Californians this week.
Note that the conjunctive adverb however always requires a comma, but don’t confuse it with however meaning “no matter how,” which doesn’t use one:
However hard we try, we can’t escape the heat or smoke.
Do you have a question about the comma? Let me know, and I’ll include it in a future installment of Mots Justes’ ongoing series.
The Mots Justes Series on Commas
Hacker, Diana, The Bedford Handbook for Writers, 3rd ed. Boston: St. Martin’s Press: 1991.