The Genitive of the Article

In the genitive the definite article is του / της / του (masculine / feminine / neuter) in the singular and των for all three genders in the plural. It is the signpost: the moment you see του, της or των in front of a noun, that phrase is a possessor or an “of” phrase, whatever the noun's own ending turns out to be - του δασκάλου, της μητέρας, των παιδιών. Two syncretisms keep the set short: masculine and neuter share του in the singular, and all three genders share των in the plural. The article is also the reliable half of the pair - a noun's genitive ending varies by class and its stress may move, but the article never does.