The Vocative Case
The least common noun case to find but easiest to learn is the vocative. The name “vocative” is from the Latin verb vocī , vocā re meaning “to call.” And that's exactly what the vocative case does — it turns nouns, usually names, into interjections (e.g., “Jeff!”).
It is the easiest of the cases to learn because the forms are identical to the nominative case with only two exceptions: the second declension masculine nominative ending -us changes to -e, and -ius changes to -ī. If you were to call Marcus, for example, you would shout “Marce!” If you needed Publius's attention, you would yell “Publī!” On the other hand, if you wanted Vipsania to notice you, the form would be “Vipsā nia!” (i.e., just like the nominative).