mutual(ly)
1.their mutual affection for each other（或one another）等表达方式里的mutual或for each other（或one another）是多余的，该改作their affection for each other（或one another）或their mutual affection。同样地，exchange，interchange，agreement，cooperation等词前面不该用mutual。下面各句里的mutually也该删去：
- They love each other mutually.
- They exchange goods mutually.
- They agree mutually.
- They cooperate mutually.
2.严格地说，mutual是“互相的”，不是“共同的”，说“共同的”该用common。如假定你爱某人，我也爱某人，那末你我两人有common love，并没有mutual love，假定你爱我，我也爱你，你我两人才有mutual love。把mutual用作“共同的”解和把mutually用作“共同地”解是英美人常有的错误。
mutual friend和mutual acquaintance用作“共同的朋友”和“共同的相识者”解照理论讲当然是错的。但这两种表达方式很普通，可以算得成立了。common friend和common acquaintance有可能被人误解作“寻常的…”。最好用a friend in common和an acquaintance in common。
