dnw, so what you are saying is that availability is different in different markets, correct? Is this the reason that when I look for flights, say from west coast USA to continental Europe or Africa (via LHR), I often find there is nothing available except in expensive Club and First. Whereas if I look at flights ex-LON then there are plenty of cheap seats?

I used to think this was just a problem with the fare explorer not seeing availability on one or more of the legs of journeys I was building, and instead giving me the most expensive options. But it is, in reality, simply a way for the airlines to control inventory and base availability according to the particular markets where a traveler originates?

Despite the fact I have been flying since I was 1 month old, I still have great trouble decoding airline fares and how they are constructed. I still do not understand, but this thread seems to clear up one or two issues (I think).