"My sister Jennifer" has a different meaning from "My sister, Jennifer." A comma tells the reader that the name "Jennifer" in no way narrows down who I mean by "my sister," so I must have only one sister. The absence of a comma tells the reader that the name is restrictive information necessary to know which sister I'm talking about. Here's the full explanation.

The post Commas for Setting Off Nonrestrictive Information first appeared on Grammar Underground with June Casagrande.