Commas set off transitional words and phrases.

• Indeed, close coordination will be essential.
• Defeat may be inevitable; however, disgrace is not.
• The second report, on the other hand, shows a strong bias.

When such words and phrases fall in the middle of a clause, commas are sometimes unnecessary.

• They thus have no chips left to bargain with.
• The materials had indeed arrived.
• She would in fact see them that afternoon.

Commas set off parenthetical elements, such as authorial asides.

• All of us, to tell the truth, were completely amazed.
  
• It was, I should add, not the first time I'd seen him in this condition.

Commas are often used to set off words or phrases that introduce examples or explanations, such as namely, for example, and that is.

• He expects to visit three countries, namely, France, Spain, and Germany.
  
• I would like to develop a good, workable plan, that is, one that would outline our goals and set a timetable for accomplishing them.
  

Such introductory words and phrases may also often be preceded by a dash, parenthesis, or semicolon. Regardless of the punctuation that precedes the word or phrase, a comma usually follows it.

• Sports develop two valuable traits—namely, self-control and the ability to make quick decisions.
  
• In writing to the manufacturer, be as specific as possible (i.e., list the missing or defective parts, describe the malfunction, and identify the store where the unit was purchased).
  
• Most had traveled great distances to participate; for example, three had come from Australia, one from Japan, and two from China.

Commas set off words in direct address.

• This is our third and final notice, Mr. Sutton.
  
• The facts, my fellow Americans, are very different.
  

Commas set off mild interjections or exclamations.

• Ah, the mosaics in Ravenna are matchless.
  
• Uh-oh, His Eminence seems to be on the warpath this morning.