Commas: They're Not Just For English Majors Anymore. commas with...

Contrasting Elements
"CE" stands for contrasting elements. Sometimes you will want to emphasize two strongly contrasting ideas or points by inserting a comma between them:
    • We throw fish at college professors, never at respectable people.
    • We should remember the lessons we've learned, not regret the time we spent on a pirate ship learning them.
    • Ahab might have forced Ishmael to walk the plank, but didn't.
Notice that in these cases (though not every case), contrasts are signaled by the words "not" and "yet."  The point is to set the contrast off for the reader.

Also notice that the second sentence has an additional strong negative, "never," which also marks the contrast.

Now try one more:

    The grandmother is a misfit, the Misfit a son.
Does the comma belong?  As before, the comma in this case underlines the contrast. So we would say yes, this comma can be justified because it signals a contrast.
Remember, the point is to justify the commas you use. Watch for key words that suggest contrast: but, not, and never are some of the most common. 

If you find one of these in your sentence, check to see if you're trying set up a strong contrast.  If so, you probably need a comma.

next (2IC)

Intro, Get Started, IE, CE, 2IC, FANBOYS, CA, NRE, Review

IE: Introductory Elements

CE: Contrasting Elements

2IC: Two Independent Clauses

CA: Compound or Coordinate Adjectives

NRE: Non-Restrictive Elements

A Few Other Situations (quotations, lists, etc.)