There is analyzed the double declension of the languages of various systems. Some cases of simple declension take additional affixes of other cases, and the word form of a noun get two case endings. Such a picture may be found in the Mongolian languages, the Yenisei languages of Siberia and in the languages of the Chukchi and Kamchatka group. The research indicated that double cases were used to express the possessive relations.

There is described the category of belonging in the Finno-Ugric languages, different ways of its expression (by genitive case, personal pronouns, possessive and personal possessive signs, verbs of possession, possessive constructions) and use.