Hillrider's explanation was so good that I thought I'd throw in a related question that recently came up for me:

Is there such a thing (recognized by fare systems) as a "double open jaw"? Example: Suppose I wanted to fly BOS-SFO and LAX-JFK, all in discount coach, all on nonstop transcons. It's easy to get a good fare BOS-LAX-JFK or BOS-LAX/SFO-BOS. But when you throw in the combination of open jaws -- open on both ends, so to speak -- the only option on the nonstop transcons is Y or B fares, or discount fares connecting in any of AA's hubs in both directions.

Double throwaway ticketing immediately comes to mind in such a situation, but that essentially doubles the itinerary's price and is of course against the rules. Could add a connection on either coast (an extra LAX-SFO segment, for example, so that the full routing becomes BOS-SFO-LAX-SFO-JFK), but that wastes time. Any way to make this price out "fairly" (presumably ~$500 r/t)?

[Edited to fix typos.]

[This message has been edited by bedelman (edited 11-22-2002).]