Separate the name of the person (or thing) you are addressing from the rest of the sentence with a comma.
What is the Vocative Case?Names that are being addressed directly are said to be in the vocative case. When somebody is being spoken to directly, his/her name must be separated from the rest of the sentence with a comma (or commas).
(Vocative stems from the word vocal.)
someone being addressed as peasant – comma is correct
Anything Can Be in the Vocative CaseAnimals, even inanimate objects, can be addressed directly. They should also be separated from the rest of the sentence to show they are in the vocative case.
Literally, this sign is telling cows to keep their dogs under control
(sign on gate to field)
COULD BE IMPORTANT
The examples below show why it is important to understand the vocative case. Whilst both are correct, they have very different meanings.
Did Bill Clinton use the vocative case to ensure his statement was "legally accurate"? The pause before he says "Miss Lewinski" could have been deliberate to ensure her name was put in the vocative case. Now, that's some smart legal counsel!
Commas with the vocative case
Commas with Dear, Hello, and Hi