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Which is proper? I have looked for an answer but have not located any info for this question. I always thought John and Mary's was correct but a friend disagrees. Is it John's and Mary's party? Or John and Mary's party?
John and Mary's party is referring to a party organized from both of them. John's and Mary's parties is referring to two different parties: one organized from John, and one from Mary.
Both are correct. Which one you use depends from the context.