The following sample sentences and the discussion that follows each point out three frequently found punctuation errors in which a comma is extraneously inserted or erroneously omitted.
1. “The giant, blue eyeball that washed up on a Florida beach likely came from a swordfish.”
The adjectives giant and blue are noncoordinate, which means they’re not parallel in function. You can say, of course, that an eyeball is giant and that it is blue, but the second test of adjectival coordination, whether the words can gracefully be transposed, does not work; “the blue giant eyeball” is awkward. Why?
A convention in English called the royal order of adjectives assigns specific starting positions to different types of descriptive words, and size precedes color. Therefore, “blue eyeball” becomes a temporary compound modified by giant, and therefore no intervening punctuation is required: “The giant blue eyeball that washed up on a Florida beach likely came from a swordfish.”
2. “Move over vampires, goblins and haunted houses, this kind of Halloween terror aims to shake up even the toughest warriors.”
The introductory phrase in this sentence, a form of address to the subject that is increasingly common in lead paragraphs in journalistic contexts (to the point of becoming a tired cliché), is just that — an introductory phrase. And though short introductory phrases are often inserted at the beginning of a sentence without following punctuation, in this case, “Move over vampires” is a miscue that readers might read to mean “proceed on top of bloodsucking beings.”
I prefer consistency over inconsistency and recommend always punctuating introductory phrases; whether you follow that advice or not, do it here: “Move over, vampires, goblins, and haunted houses, this kind of Halloween terror aims to shake up even the toughest warriors.”
3. “The convention will be delayed until Tuesday because of the threat of the tropical storm Isaac now bearing down on Florida.”
There are at least two effective solutions to the problem here, which is that “angry tropical storm” and Isaac are appositives, which means that one noun or noun phrase refers to the other. As written, without punctuation, the sentence implies that more than one angry tropical storm bearing down on Florida exists at this time, and one is called Isaac.
But because only one storm, named Isaac (“one storm” and “named Isaac” are in apposition), is bearing down on Florida, the interchangeable noun and noun phrase are set off with an appositive comma: “The convention will be delayed until Tuesday because of the threat of Isaac, the tropical storm now bearing down on Florida.”
Another option is to refer to Isaac with the modifying phrase “tropical storm” and follow the wording with a descriptive phrase, set off by a comma, that serves an appositive function: “The convention will be delayed until Tuesday because of the threat of the tropical storm Isaac, now bearing down on Florida.”