Web sites including Points.com and LoyaltyMatch.com let people trade frequent-flier miles from one airline for miles in another. The miles can also be traded for hotel-reward points, merchandise or cash. How they work: On Points.com's GPX exchange, users post trade offers including the airline and number of miles they wish to give up, and the airline and number of miles they want in return. They can also browse potential trades by airline, and click to accept a trade. Cost: There is no charge for listing a trade, but Points.com charges both parties when a trade is completed. Fees vary with the airline from which miles are given up and the number of miles. For example, trading away 10,000 miles costs $130 on American or Delta and $100 on Continental. Exchange rates: Chris Barnard, president of Points.com, says most trades are 1-for-1 — each side gets the same number of miles. But not always. This week there were several offers to exchange Delta miles for fewer American miles. Some looked like feeler offers, including a 30-for-1 swap that had not been accepted by any other user. Not all airlines covered: Ten airlines allow miles to be traded on Points.com: Aeroplan, AirTran, Alaska, American, Continental, Delta, Frontier, Icelandair, Midwest and TACA, plus the Intercontinental chain of hotels. Your program: Check the terms and conditions to see whether your airline lets you trade or give your miles to someone else. Most airlines state that their miles cannot be purchased, sold or bartered outside approved channels.