Detailed example of a destination open-jaw
In this example we will discuss the math behind "destination open jaw" tickets.
The framework is A->B, C->A: in our example we are flying Seattle to SE Asia city 1 and back from SE Asia city 2 to Seattle.
One key rule for an open jaw flight is that the distance between the "missing leg" (B->C) needs to be less than both other legs of the flight (A->B and C->A). Because you are doing open jaws across the pacific ocean you are in great shape as the distance B->C is going to be much smaller than A->B and C->A. Here is a crude diagram I made for possible destination open jaws: (Note the diagram changes slightly for origin open jaws and for the double open jaw -- exercise for the reader is to draw this diagram for the double open jaw :-P).
A = Seattle.
B = Bangkok.
Placing another city C that is within the green region around B will ensure that BC > AB and BC < AC.
So why are open jaws cheap flights?
US carriers price most international flights as roundtrip tickets. So when buying internationally most of the time it's considerably cheaper to purchase a roundtrip ticket rather than buying two oneways. (Side note: there are even cases where it is cheaper to purchase a roundtrip ticket than purchasing a oneway ticket). This is where open jaws come in!
When you purchase an open jaw flight you are getting the roundtrip ticket pricing for oneway tickets. More specifically you get each oneway priced at half the price of what a roundtrip would cost. An example will help explain this better.
Example: Flying Seattle to Phnom Penh then Bangkok to Seattle is $802 together as an open jaw but booking as oneways is $597 + $873 = $1,470. Here the open jaw saves a lot of money!
In this example the price of the open jaw comes from:
1/2 * (price of a roundtrip seattle to Phnom Penh + price of a roundtrip Seattle to Bangkok).
(Note the prices here are from flights in September 2016)
