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The Smiths decided to move to the country so that they could live in peace and quiet.

Hi,

Is "and quiet" optional in the above? Thanks.
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No. Peace and quiet is an idiomatic phrase meaning 'away from noise and activity; away form the hurly-burly'. Merely using 'live in peace' suggests that the Smiths are in danger, or are being persecuted or bothered elsewhere.
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Hi Angliholic

"Peace and quiet" is an extremely common collocation. To me, the word 'quiet' is an important part of the description. So, for those two reasons, I would not consider 'and quiet' to be optional.

EDIT:
Oops, I see MM was much faster than I was. His description of how the word 'peace' (alone) would be interpreted is just right.
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Thanks, Mister.

Got it.
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And thank you, Amy.