Where does an apostrophe go in a possessive?

An apostrophe is placed before the "s" to indicate possession for singular nouns (e.g., "the dog's collar"). For plural nouns that already end in "s," the apostrophe is added after the "s" (e.g., "the dogs' park"). For plural nouns that do not end in "s," the apostrophe precedes the "s" (e.g., "the children's toys").