Bryan Garner’s Usage Tip of the Day concerns the word mutual :
It’s possible to refer to a couple’s mutual devotion, but not their mutual devotion to their children. The reason is that whatever is “mutual” is reciprocal — it’s directed by each toward the other. E.g.: “So consider the matter a quid pro quo, a mutual exchange of affection between Zereoue and Mountaineer fandom.” Michael Dobie, “More-Famous Amos,” Newsday (N.Y.), 14 Nov. 1997, at A103.Today’s tip is well-timed: Elaine and I just started Our Mutual Friend.
But when the sense is “shared by two or more,” then the word is “common” — not “mutual.” So “friend in common” is preferable to “mutual friend,” although the latter has stuck because of Dickens’s novel Our Mutual Friend (the title to which, everyone forgets, comes from a sentence said by an illiterate character).
Careful writers continue to use “friend in common.”
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