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The possessive form is used in a prepositional phrase beginning with 'of':
'Maggie, a colleague of Mary's, came to the opening of the exhibition. Rufus came too.' 'Who's Rufus?' 'He's a friend of my husband's.'
What do Mary's and husband's mean in these sentences? At first I thought "He's a friend of my husband's" could mean "He is a friend of friend of my husband" but I am not sure.