Originally Posted by
javabytes
If the routing rules for BOS-ZAG only allow BOS-CDG/AMS-ZAG, then no, you can't. The fact that you connected in CDG doesn't open any other doors for you. There can be all sorts of other routing rules for CDG-ZAG, possibly including through PRG, but it doesn't matter, since your fare (assuming you're not breaking the fare) is BOS-ZAG and those are the routing rules you must follow.
So to eschew any further obfuscation, the rule can be simply stated as "A stopover can only occur in a city that is part of a published fare between the origin and the destination [i.e. point of turnaround] or between the point of turnaround and the final stop of the itinerary [i.e. could be origin or another city via last leg open jaw]."
Javabytes, is this also true of layovers, or are the rules relaxed for those?