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APPOSITION

Words in nonrestrictive apposition are set off with commas; those that restrict preceding material are not.

1.Nonrestrictive apposition merely adds information to something already identified:
        John's play, Phalanges, was a flop. (John wrote only one play.)

2.Restrictive apposition completes the identification:
        Shakespeare's play Macbeth is on the reading list (one of many plays by Shakespeare).

3.Relationships take commas or not depending upon how many relatives of the same kind there are. If there is but one, use commas:
        my wife, Nellie, and my son, Bob, came too (only one wife and only one son)
        Bob and his wife, Ann; but Bob and wife Ann.

If there are two or more, no commas:
        her sister Millie and her cousin Karen came too (more than one sister and more than one cousin)