Why might a multi-leg flight be cheaper than one of its legs? [duplicate]

I expect this has been asked and answered elsewhere, but I couldn't find an exact duplicate.

The multi-leg return fare:

CWL > AMS > PDX > AMS > CWL

is significantly cheaper than:

AMS > PDX > AMS

KLM's booking office couldn't explain to me exactly why, other than to say it's how the fare structure works.

I assume the airlines have good economic reasons for this, though I can't imagine what they are.

And I also assume that the fare system and its terms and conditions have been carefully engineered, for whatever reason, to make a cheaper fare for the single leg unavailable.

fare-basis

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Is it cheaper to take multiple flights?

Booking a multi-city flight may seem like it would be more expensive than a roundtrip flight, but it's typically not. In fact, booking a multi-city itinerary is often much more affordable than booking two one-way flights.

Why are layovers cheaper?

In a very basic sense, the airlines try and forecast supply and demand as best as they can for every single ticket, and then price each one accordingly so as to sell for the highest possible profit. This results in price discrimination, or charging different prices for the equivalent seat on an airplane.

What happens if you only take one leg of a flight?

"Normally what happens is if you miss one leg, your entire itinerary is cancelled," he said over the phone. "That's just a standard practice.

What does multiple fares mean on flights?

Airlines often sell multiple fare classes at different prices, with a couple seats in each class. If there's only one seat left in the lowest fare class and you search for four seats, most automated systems will show you the highest fare class for all four tickets.

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