Q. My son will be spending next semester in Europe and I am trying to find flights for him. I've discovered in several instances where flights from the U.S. on SAS are at least $100 cheaper than the identical flights booked through its partner United. Why is there this differential and, more importantly, is there any reason why I should book through United instead of SAS?

A. This is a code share flight and there's no reason why you shouldn't buy the fare from whoever is selling it for less. Airlines buy blocks of seats on other airlines for a set price and then are free to sell them for whatever they wish. We've seen fare differences on various routes for much more than $100 in the past. A good way to identify code shared flights is by using Orbitz.com, which will sometimes show you the exact same flights at different prices on different airlines.