The Sociative case reference article from the English Wikipedia on 24-Apr-2004
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Sociative case

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Cases
Abessive case
Ablative case
Absolutive case
Accusative case
Adessive case
Allative case
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Dative case
Dedative case
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Ergative case
Essive case
Genitive case
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Inessive case
Instrumental case
Locative case
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Oblique case
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Declension
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This case in Hungarian language can express the person in whose company (cf. Latin socius) the action is carried out, or to any belongings of people which take part in the action (together with their owners). This case is not productive any more; nowadays the Instrumental-comitative case is usually used instead.