Possessive pronouns.
Whose is it?

Possessive pronouns answer one question: whose? mein, dein, sein, ihr, unser, euer, and ihr · Ihr each point back to a person: my, your, his, her, our, your, their. But the ending on the possessive pronoun has nothing to do with that person. It agrees with the noun that follows, exactly the way ein and kein do.

One possessive pronoun per owner

Every personal pronoun has a matching possessive pronoun. The base form below is the one used before a masculine or neuter noun in the Nominativ.

Owner	Possessive pronoun	English
ich	mein	my
du	dein	your (informal)
er	sein	his
sie	ihr	her
es	sein	its
wir	unser	our
ihr	euer	you all (informal)
sie · Sie	ihr · Ihr	their / your (formal)

sein covers both er and es, and ihr covers sie (singular and plural) as well as the formal Sie, capitalized as Ihr. The owner decides which word to pick. The noun that follows decides everything else.

The pronouns these are built from: Personal pronouns. The Nominativ set.

The ending matches the noun, not the owner

A possessive pronoun behaves like ein or kein: it adds an ending depending on the gender, number, and case of the noun that follows. In the Nominativ, masculine and neuter nouns take no extra ending at all. Feminine nouns and plurals add -e.

Gender	Nominativ	Example
masculine	mein	mein Bruder (my brother)
feminine	meine	meine Schwester (my sister)
neuter	mein	mein Kind (my child)
plural	meine	meine Eltern (my parents)

Across the cases: meinen, meinem, meiner

Once a possessive pronoun's noun becomes a direct object or a recipient, the ending changes again, the same way der becomes den or dem. Masculine nouns change in both the Akkusativ and the Dativ. Feminine, neuter, and plural nouns keep their Nominativ form in the Akkusativ, and only change in the Dativ.

Gender / number	Nominativ	Akkusativ	Dativ
masculine (Bruder)	mein	meinen	meinem
feminine (Schwester)	meine	meine	meiner
neuter (Kind)	mein	mein	meinem
plural (Eltern)	meine	meine	meinen

In a sentence → Mein Bruder wohnt in Berlin. (My brother lives in Berlin, Nominativ subject.) Ich besuchemeinen Bruder. (I visit my brother, Akkusativ object.) Ich schreibemeinem BrudereineE-Mail. (I write my brother an email, Dativ recipient.) The pronoun mein changes shape three times. The owner, "I," never does.

wessen? finds the owner

wessen? means "whose?" It is the question word for ownership, and the answer is usually a possessive pronoun plus a noun, or a name in the Genitiv.

• Wessen Buch ist das? → Das istmein Buch. (Whose book is this? This is my book.)
• Wessen Auto steht hier? → Das istihr Auto. (Whose car is parked here? That's her car.)

The question word in full: wessen? The Genitiv question word.

For the bigger picture, including how pronouns change with case and where they sit in a sentence: Pronomen. Words that replace nouns.