It's not restrictive, is it? It's actually parenthetical in nature. Here's the rule:

The important point to remember is that a nonessential appositive is always separated from the rest of the sentence with comma(s).

"I'd like you to meet my wife, Peggy."

"I'd like you to meet my brother Bob."

Wife and Peggy are separated by a comma because Peggy is not strictly necessary to the sentence because people generally only have one wife. However, I might have five brothers, so the absence of the comma signals the importance of the bond between brother and Bob.

The Gospel according to http://www.chompchomp.com/terms as well as a "host" of other sites.






TEd