NEW YORK (Money) -- QUESTION: My husband and I are fencing in our backyard. We'd like to connect our fence to our next door neighbor's existing fence -- in other words, have one common fence (theirs) separating the two properties rather than two parallel fences.

If the neighbors let us do this, should we offer to pay something toward the cost of their fence? We'll have saved a few thousand dollars, and, if it matters, their fence is only two years old.

Money Magazine's ethicists are consultants who advise attorneys on people's ethical beliefs. E-mail them at right_thing@moneymail.com.

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ANSWER: That's the trouble with poets. Robert Frost tells us that good fences make good neighbors, but he doesn't say who should pay for the fence.

So here's the deal: When they built the fence, did your neighbors ask you to split the cost or hint that they'd like you to? Because if they did, you need to pay your share of their fence's construction cost if you expect to piggyback onto it.

Similarly, if your neighbors consulted with you when they built the fence and made it a point to accommodate your preferences, you should offer to pay a portion of its cost.

After all, you'd be having to spend several thousand dollars on a parallel fence if they had built one you didn't care for. Only if your neighbors built the fence without any consideration for your preferences are you off the hook. Even then, though, if your neighbors say "yes" to your request, you should take them out to an elegant dinner or send them a case of good wine.

A favor that saves you thousands of dollars merits a serious display of appreciation, even if the favor cost your neighbors nothing to grant.

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