SAN FRANCISCO - For some, it's the red reindeer Christmas sweater. For others, it's the diamond-encrusted dreidel.

Whatever your worst gift nightmare might be, Amazon may soon give you a reason to cheer.

Amazon.com Inc., the source of many presents good, bad, or ugly, has patented a system that would let people exchange unwanted gifts for those they actually want - even before they receive them.

For now, though, it's just an idea, so until Amazon figures out how to implement it, you may be stuck with that itchy sweater from Grandma, even if she bought it online.

Amazon did not return messages Wednesday to speak about its plans.

Based on Amazon's patent filing, recipients of unwanted gifts would be able to exchange Amazon-bought items for something of equal value, pay the difference for a more expensive item, or get a gift certificate.

They could do this before they actually receive the gift, thus saving themselves the hassle of repackaging and mailing the unwanted present.

If they wish, recipients could even send a thank-you note for the original gift. They can disclose to the sender that their gift has been converted, or simply keep it from them and just hope they don't ask about it later.

If there's a friend or family member with a history of giving unwanted presents - Amazon's patent filing calls her "Aunt Mildred" - users could select to convert all gifts from this person into something else.

Amazon says its idea would benefit gift-givers as well, as they can take a chance on a more interesting gift knowing that it can be returned if the recipient chooses.

Also, a charity could use the idea to streamline donations, according to the patent filing. For example, a children's charity asking for mittens could have donations stop once they have 100 pairs. The rest can be converted into hats, scarves, or gift certificates.

Amazon, which is based in Seattle, applied for the patent in March 2006 and received it Nov. 9.

Even if Amazon implements the system, it's unclear how many people would use it.

In part because returning gifts purchased online is such a hassle, people who buy presents this way generally make sure it's something the recipient really wants, said C. Britt Beemer, chairman of America's Research Group, a consumer and market research company.

Or they give gift cards.

A decade ago, before gift cards became ubiquitous, about 38 percent of consumers returned presents after Christmas, Beemer said. Last year, this was only about 13 percent. The percentage is even smaller for gifts bought online.