- The word is commonly used with a definite article in most parts of central Germany, southern Germany, and Austria: Wo ist der Vati? – "Where is Dad?" In northern Germany—and generally in writing—no article tends to be used: Wo ist Vati?
- The genitive Vatis is used before the modified term, without any article or determiner: Vatis Tasche – “Dad’s bag”. This is chiefly northern German usage; the rest of the language prefers: die Tasche vom Vati (as above). — The genitive can also be used with an accompanying article or determiner: die Tasche meines Vatis – “my dad’s bag”. This, however, is not common usage in any region; children will say instead: die Tasche von meinem Vati.
- Vati in Duden online