Please note: Latin Declension Rules
Example of the Vocative:Et tu, Brute
And you, Brutus?
The vocative case is used to address someone by name, much as we would say in English, "Peter, please pass the pepper" or "Please pass the pepper, Peter." The comma after or before the name serves to set off the person addressed from the rest of the sentence. Latin uses a separate case for this function -- the vocative. In most instances, the case of the vocative is indistinguishable from the nominative, but in the masculine singular of certain second declension nouns, the "-us" ending becomes "-e" (as in "Et tu, Brute") and the "-ius" ending becomes "-i."