Phrases Prepositional phrases are usually restrictive, but sometimes they are not essential to the meaning of a sentence and are therefore set off with commas. Appositives An appositive is a noun or noun phrase that renames a nearby noun. When an appositive is not essential to identify what it renames, it is set off with commas. The next to last item in a series may often appear without a comma. Omitting this comma may cause confusion, however, and you will never be wrong if you include it.
Series commas are quite often a matter of style.
Use commas to set off parenthetical and transitional expressions. Parenthetical expressions add comments or information. They often interrupt the flow of a sentence or digress, they are usually set off with commas. Use commas to set off contrasting elements, interjections, direct address, and tag questions. Use commas to set off parts of dates, addresses, titles, and numbers.