Quick travel quiz: You've booked a westbound flight on what you thought was United Airlines. Suddenly, you find yourself boarding a Delta plane. You:

A. grabbed the wrong ticketB. took the wrong tram in the terminal

C. will be heading eastward if you don't make a quick U-turn.

Answer: Probably none of the above. Chances are you're flying Delta because that airline and United have reached an agreement to buy blocks of seats on each other's planes and sell tickets on the same flights.

Continental and Northwest, and American and US Airways, have announced similar relationships, all of which are to be phased in by next year. Such agreements, known as code sharing, let airlines expand networks and increase revenues without adding new routes or merging with other carriers.

That benefits travelers because it increases the number of available destinations on your preferred carrier and lets you earn and redeem frequent-flier miles on more flights.

Along with the expanded routes, however, travelers should also "expect increased confusion and inconvenience when making connections that require a change in airlines," says Tom Parsons, editor of Best Fares Discount Travel Mag-azine (www.bestfares.com).

A connecting flight may depart from a different terminal or, on foreign flights, you may discover that although you booked a flight on a major U.S. airline, your seat is actually on a carrier from South America or Ukraine.

When booking a flight, ask if the flight is a code share and which airlines are participating. Code sharing carriers often price their tickets differently.