both in latin and in greek, the declension (inflection) of some nouns uses a different stem in the oblique cases than in the nominative and vocative singular cases. such words belong to, respectively, the so-called third declension of the latin grammar and the so-called third declension of the ancient greek grammar. for example, the genitive singular is formed by adding "-is" (latin) or -ος (greek) to the oblique stem.